added a comment - - edited I am not sure but I think this was the copy of this bug description. I don't have it handy anymore.
My system is Windows 10 and I am using a MariaDB made from source.
I had seen this once in a while but very rarely. No wonder it is difficult to reproduce.
About the cnf file, I don't know where it is? How can I find this?
Variable_name,Value
aria_block_size,8192
aria_checkpoint_interval,30
aria_checkpoint_log_activity,1048576
aria_encrypt_tables,OFF
aria_force_start_after_recovery_failures,0
aria_group_commit,none
aria_group_commit_interval,0
aria_log_file_size,1073741824
aria_log_purge_type,immediate
aria_max_sort_file_size,9223372036853727232
aria_page_checksum,ON
aria_pagecache_age_threshold,300
aria_pagecache_buffer_size,134217728
aria_pagecache_division_limit,100
aria_pagecache_file_hash_size,512
aria_recover,NORMAL
aria_repair_threads,1
aria_sort_buffer_size,268434432
aria_stats_method,nulls_unequal
aria_sync_log_dir,NEWFILE
aria_used_for_temp_tables,ON
auto_increment_increment,1
auto_increment_offset,1
autocommit,ON
automatic_sp_privileges,ON
back_log,80
basedir,C:\MariaDB-10.1\MariaDB\bld\sql\
big_tables,OFF
binlog_annotate_row_events,OFF
binlog_cache_size,32768
binlog_checksum,NONE
binlog_commit_wait_count,0
binlog_commit_wait_usec,100000
binlog_direct_non_transactional_updates,OFF
binlog_format,STATEMENT
binlog_optimize_thread_scheduling,ON
binlog_row_image,FULL
binlog_stmt_cache_size,32768
bulk_insert_buffer_size,8388608
character_set_client,latin1
character_set_connection,latin1
character_set_database,latin1
character_set_filesystem,binary
character_set_results,
character_set_server,latin1
character_set_system,utf8
character_sets_dir,C:\MariaDB-10.1\MariaDB\bld\sql\share\charsets\
collation_connection,latin1_swedish_ci
collation_database,latin1_swedish_ci
collation_server,latin1_swedish_ci
completion_type,NO_CHAIN
concurrent_insert,AUTO
connect_class_path,
connect_conv_size,8192
connect_exact_info,OFF
connect_indx_map,OFF
connect_java_wrapper,wrappers/JdbcInterface
connect_json_grp_size,10
connect_jvm_path,
connect_timeout,10
connect_type_conv,NO
connect_use_tempfile,AUTO
connect_work_size,67108864
connect_xtrace,0
datadir,C:\MariaDB-10.1\MariaDB\bld\sql\data\
date_format,%Y-%m-%d
datetime_format,%Y-%m-%d %H:%i:%s
deadlock_search_depth_long,15
deadlock_search_depth_short,4
deadlock_timeout_long,50000000
deadlock_timeout_short,10000
debug,
debug_binlog_fsync_sleep,0
debug_dbug,
debug_no_thread_alarm,OFF
debug_sync,OFF
default_master_connection,
default_regex_flags,
default_storage_engine,MyISAM
default_tmp_storage_engine,
default_week_format,0
delay_key_write,ON
delayed_insert_limit,100
delayed_insert_timeout,300
delayed_queue_size,1000
div_precision_increment,4
encrypt_binlog,OFF
encrypt_tmp_disk_tables,OFF
encrypt_tmp_files,OFF
enforce_storage_engine,
error_count,0
event_scheduler,OFF
expensive_subquery_limit,100
expire_logs_days,0
explicit_defaults_for_timestamp,OFF
external_user,
extra_max_connections,1
extra_port,0
flush,OFF
flush_time,0
foreign_key_checks,ON
ft_boolean_syntax,+ -><()~*:""&|
ft_max_word_len,84
ft_min_word_len,4
ft_query_expansion_limit,20
ft_stopword_file,(built-in)
general_log,OFF
general_log_file,VENELLE.log
group_concat_max_len,1024
gtid_binlog_pos,
gtid_binlog_state,
gtid_current_pos,
gtid_domain_id,0
gtid_ignore_duplicates,OFF
gtid_seq_no,0
gtid_slave_pos,
gtid_strict_mode,OFF
have_compress,YES
have_crypt,NO
have_dynamic_loading,YES
have_geometry,YES
have_openssl,NO
have_profiling,YES
have_query_cache,YES
have_rtree_keys,YES
have_ssl,DISABLED
have_symlink,NO
histogram_size,0
histogram_type,SINGLE_PREC_HB
host_cache_size,279
hostname,VENELLE
identity,0
ignore_builtin_innodb,OFF
ignore_db_dirs,
in_transaction,0
init_connect,
init_file,
init_slave,
insert_id,0
interactive_timeout,28800
join_buffer_size,262144
join_buffer_space_limit,2097152
join_cache_level,2
keep_files_on_create,OFF
key_buffer_size,134217728
key_cache_age_threshold,300
key_cache_block_size,1024
key_cache_division_limit,100
key_cache_file_hash_size,512
key_cache_segments,0
large_files_support,ON
large_page_size,0
large_pages,OFF
last_gtid,
last_insert_id,0
lc_messages,en_US
lc_messages_dir,
lc_time_names,en_US
license,GPL
local_infile,ON
lock_wait_timeout,31536000
log_bin,OFF
log_bin_basename,
log_bin_index,
log_bin_trust_function_creators,OFF
log_error,
log_output,FILE
log_queries_not_using_indexes,OFF
log_slave_updates,OFF
log_slow_admin_statements,OFF
log_slow_filter,admin,filesort,filesort_on_disk,full_join,full_scan,query_cache,query_cache_miss,tmp_table,tmp_table_on_disk
log_slow_rate_limit,1
log_slow_slave_statements,OFF
log_slow_verbosity,
log_tc_size,24576
log_warnings,1
long_query_time,10.000000
low_priority_updates,OFF
lower_case_file_system,ON
lower_case_table_names,1
master_verify_checksum,OFF
max_allowed_packet,4194304
max_binlog_cache_size,18446744073709547520
max_binlog_size,1073741824
max_binlog_stmt_cache_size,18446744073709547520
max_connect_errors,100
max_connections,151
max_delayed_threads,20
max_digest_length,1024
max_error_count,64
max_heap_table_size,16777216
max_insert_delayed_threads,20
max_join_size,18446744073709551615
max_length_for_sort_data,1024
max_long_data_size,4194304
max_prepared_stmt_count,16382
max_relay_log_size,1073741824
max_seeks_for_key,4294967295
max_session_mem_used,9223372036854775807
max_sort_length,1024
max_sp_recursion_depth,0
max_statement_time,0.000000
max_tmp_tables,32
max_user_connections,0
max_write_lock_count,4294967295
metadata_locks_cache_size,1024
metadata_locks_hash_instances,8
min_examined_row_limit,0
mrr_buffer_size,262144
multi_range_count,256
myisam_block_size,1024
myisam_data_pointer_size,6
myisam_max_sort_file_size,2146435072
myisam_mmap_size,18446744073709551615
myisam_recover_options,DEFAULT
myisam_repair_threads,1
myisam_sort_buffer_size,134216704
myisam_stats_method,NULLS_UNEQUAL
myisam_use_mmap,OFF
mysql56_temporal_format,ON
named_pipe,OFF
net_buffer_length,16384
net_read_timeout,30
net_retry_count,10
net_write_timeout,60
old,OFF
old_alter_table,OFF
old_mode,
old_passwords,OFF
open_files_limit,6210
optimizer_prune_level,1
optimizer_search_depth,62
optimizer_selectivity_sampling_limit,100
optimizer_switch,index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,extended_keys=on,exists_to_in=on,orderby_uses_equalities=off
optimizer_use_condition_selectivity,1
performance_schema,OFF
performance_schema_accounts_size,-1
performance_schema_digests_size,-1
performance_schema_events_stages_history_long_size,-1
performance_schema_events_stages_history_size,-1
performance_schema_events_statements_history_long_size,-1
performance_schema_events_statements_history_size,-1
performance_schema_events_waits_history_long_size,-1
performance_schema_events_waits_history_size,-1
performance_schema_hosts_size,-1
performance_schema_max_cond_classes,80
performance_schema_max_cond_instances,-1
performance_schema_max_digest_length,1024
performance_schema_max_file_classes,50
performance_schema_max_file_handles,32768
performance_schema_max_file_instances,-1
performance_schema_max_mutex_classes,200
performance_schema_max_mutex_instances,-1
performance_schema_max_rwlock_classes,40
performance_schema_max_rwlock_instances,-1
performance_schema_max_socket_classes,10
performance_schema_max_socket_instances,-1
performance_schema_max_stage_classes,150
performance_schema_max_statement_classes,178
performance_schema_max_table_handles,-1
performance_schema_max_table_instances,-1
performance_schema_max_thread_classes,50
performance_schema_max_thread_instances,-1
performance_schema_session_connect_attrs_size,-1
performance_schema_setup_actors_size,100
performance_schema_setup_objects_size,100
performance_schema_users_size,-1
pid_file,C:\MariaDB-10.1\MariaDB\bld\sql\data\VENELLE.pid
plugin_dir,C:\MariaDB-10.1\MariaDB\bld\storage\connect\Debug\
plugin_maturity,unknown
port,3306
preload_buffer_size,32768
profiling,OFF
profiling_history_size,15
progress_report_time,5
protocol_version,10
proxy_user,
pseudo_slave_mode,OFF
pseudo_thread_id,3
query_alloc_block_size,16384
query_cache_limit,1048576
query_cache_min_res_unit,4096
query_cache_size,1048576
query_cache_strip_comments,OFF
query_cache_type,OFF
query_cache_wlock_invalidate,OFF
query_prealloc_size,24576
rand_seed1,124987279
rand_seed2,1038881015
range_alloc_block_size,4096
read_buffer_size,131072
read_only,OFF
read_rnd_buffer_size,262144
relay_log,
relay_log_basename,
relay_log_index,
relay_log_info_file,relay-log.info
relay_log_purge,ON
relay_log_recovery,OFF
relay_log_space_limit,0
replicate_annotate_row_events,OFF
replicate_do_db,
replicate_do_table,
replicate_events_marked_for_skip,REPLICATE
replicate_ignore_db,
replicate_ignore_table,
replicate_wild_do_table,
replicate_wild_ignore_table,
report_host,
report_password,
report_port,3306
report_user,
rowid_merge_buff_size,8388608
secure_auth,ON
secure_file_priv,
server_id,0
shared_memory,OFF
shared_memory_base_name,MYSQL
skip_external_locking,ON
skip_name_resolve,OFF
skip_networking,OFF
skip_parallel_replication,OFF
skip_replication,OFF
skip_show_database,OFF
slave_compressed_protocol,OFF
slave_ddl_exec_mode,IDEMPOTENT
slave_domain_parallel_threads,0
slave_exec_mode,STRICT
slave_load_tmpdir,C:\Users\UTILIS~1\AppData\Local\Temp
slave_max_allowed_packet,1073741824
slave_net_timeout,3600
slave_parallel_max_queued,131072
slave_parallel_mode,conservative
slave_parallel_threads,0
slave_run_triggers_for_rbr,NO
slave_skip_errors,OFF
slave_sql_verify_checksum,ON
slave_transaction_retries,10
slave_type_conversions,
slow_launch_time,2
slow_query_log,OFF
slow_query_log_file,VENELLE-slow.log
socket,MySQL
sort_buffer_size,2097152
sql_auto_is_null,OFF
sql_big_selects,ON
sql_buffer_result,OFF
sql_log_bin,ON
sql_log_off,OFF
sql_mode,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
sql_notes,ON
sql_quote_show_create,ON
sql_safe_updates,OFF
sql_select_limit,18446744073709551615
sql_slave_skip_counter,0
sql_warnings,OFF
ssl_ca,
ssl_capath,
ssl_cert,
ssl_cipher,
ssl_crl,
ssl_crlpath,
ssl_key,
storage_engine,MyISAM
stored_program_cache,256
strict_password_validation,ON
sync_binlog,0
sync_frm,ON
sync_master_info,10000
sync_relay_log,10000
sync_relay_log_info,10000
system_time_zone,Paris, Madrid (heure d?été
table_definition_cache,400
table_open_cache,2000
thread_cache_size,0
thread_concurrency,10
thread_handling,pool-of-threads
thread_pool_max_threads,1000
thread_pool_min_threads,1
thread_stack,295936
time_format,%H:%i:%s
time_zone,SYSTEM
timed_mutexes,OFF
timestamp,1495038524.568276
tmp_table_size,16777216
tmpdir,C:\Users\UTILIS~1\AppData\Local\Temp
transaction_alloc_block_size,8192
transaction_prealloc_size,4096
tx_isolation,REPEATABLE-READ
tx_read_only,OFF
unique_checks,ON
updatable_views_with_limit,YES
use_stat_tables,NEVER
userstat,OFF
version,10.1.23-MariaDB-debug
version_comment,Source distribution
version_compile_machine,x64
version_compile_os,Win64
version_malloc_library,system
version_ssl_library,YaSSL 2.4.2
wait_timeout,28800
warning_count,0
[note] I tried to reproduce on 10.1.22, Ubuntu 16.04. and got proper error message ==Got error 122 'Field 2 too long for m line== in both cases.