Multiple crashes on Galera 5-nodes after simple ALTER-statement

While update a simple table with an simple alter statement the 5-node multimaster Galera stops working an the table gets corrupted. This has happened several times nog with different tables and simple alterstatements (without triggers).

The errorlog shows this:

2019-01-15 10:47:19 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) synced with group.
2019-01-15 11:07:45 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) desyncs itself from group
2019-01-15 11:07:46 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) resyncs itself to group
2019-01-15 11:07:46 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) synced with group.
2019-01-15 11:27:40 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) desyncs itself from group
2019-01-15 11:27:41 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) resyncs itself to group
2019-01-15 11:27:41 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) synced with group.
2019-01-15 11:47:23 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) desyncs itself from group
2019-01-15 11:47:24 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) resyncs itself to group
2019-01-15 11:47:24 140487941920512 [Note] WSREP: Member 1.0 (elavsu6211.company.local) synced with group.
2019-01-15 12:24:39 140452405958400 [Note] WSREP: MDL BF-BF conflict

schema: databasename
request: (8227134 seqno 46874664 wsrep (2, 1, 0) cmd 3 3 ALTER TABLE `subject_bpztools_aagenda` ADD `id_subject_cat` int(11) NULL DEFAULT '0' AFTER `id_subject`, ADD INDEX `id_s$
granted: (15 seqno 46874665 wsrep (1, 0, 0) cmd 0 147 (null))
2019-01-15 12:24:40 140452405958400 [Note] WSREP: MDL BF-BF conflict
schema: databasename
request: (8227134 seqno 46874664 wsrep (2, 1, 0) cmd 3 3 ALTER TABLE `subject_bpztools_aagenda` ADD `id_subject_cat` int(11) NULL DEFAULT '0' AFTER `id_subject`, ADD INDEX `id_s$
granted: (15 seqno 46874665 wsrep (1, 0, 0) cmd 0 147 (null))
2019-01-15 12:24:40 140452405958400 [Note] WSREP: MDL BF-BF conflict
schema: databasename
request: (8227134 seqno 46874664 wsrep (2, 1, 0) cmd 3 3 ALTER TABLE `subject_bpztools_aagenda` ADD `id_subject_cat` int(11) NULL DEFAULT '0' AFTER `id_subject`, ADD INDEX `id_s$
granted: (11 seqno 46874666 wsrep (1, 0, 0) cmd 0 147 (null))
2019-01-15 12:24:40 0x7fbd9fc3d700 InnoDB: Assertion failure in file /home/buildbot/buildbot/padding_for_CPACK_RPM_BUILD_SOURCE_DIRS_PREFIX/mariadb-10.2.14/storage/innobase/row/row0merge.cc l$

InnoDB: Failing assertion: table->get_ref_count() == 0
InnoDB: We intentionally generate a memory trap.
InnoDB: Submit a detailed bug report to https://jira.mariadb.org/
InnoDB: If you get repeated assertion failures or crashes, even
InnoDB: immediately after the mysqld startup, there may be
InnoDB: corruption in the InnoDB tablespace. Please refer to
InnoDB: https://mariadb.com/kb/en/library/xtradbinnodb-recovery-modes/
InnoDB: about forcing recovery.

190115 12:24:40 [ERROR] mysqld got signal 6 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,or misconfigured. This error can also be caused by malfunctioning hardware.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed,
something is definitely wrong and this may fail.
Server version: 10.2.14-MariaDB-log
key_buffer_size=134217728
read_buffer_size=131072
max_used_connections=837
max_threads=1502
thread_count=280

It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 3431472 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x7fbe2d906c18
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went terribly wrong...
stack_bottom = 0x7fbd9fc3cd80 thread_stack 0x49000
/usr/sbin/mysqld(my_print_stacktrace+0x2b)[0x55f4e00d8fab]
/usr/sbin/mysqld(handle_fatal_signal+0x535)[0x55f4dfbad005]
/lib64/libpthread.so.0(+0xf7e0)[0x7fc5f97f67e0]
/lib64/libc.so.6(gsignal+0x35)[0x7fc5f7e50495]
/lib64/libc.so.6(abort+0x175)[0x7fc5f7e51c75]
/usr/sbin/mysqld(+0x47c4eb)[0x55f4df97a4eb]
/usr/sbin/mysqld(+0x90edcc)[0x55f4dfe0cdcc]
/usr/sbin/mysqld(+0x873236)[0x55f4dfd71236]
/usr/sbin/mysqld(_Z17mysql_alter_tableP3THDPcS1_P14HA_CREATE_INFOP10TABLE_LISTP10Alter_infojP8st_orderb+0x29ed)[0x55f4dfab181d]
/usr/sbin/mysqld(_ZN19Sql_cmd_alter_table7executeEP3THD+0x3ae)[0x55f4dfaf62fe]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0xf81)[0x55f4dfa2b251]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_statebb+0x29a)[0x55f4dfa327ca]
/usr/sbin/mysqld(+0x5348c0)[0x55f4dfa328c0]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcjbb+0x18cd)[0x55f4dfa346fd]
/usr/sbin/mysqld(_Z10do_commandP3THD+0x16e)[0x55f4dfa350ee]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP7CONNECT+0x16f)[0x55f4dfaf335f]
/usr/sbin/mysqld(handle_one_connection+0x44)[0x55f4dfaf3484]
/lib64/libpthread.so.0(+0x7aa1)[0x7fc5f97eeaa1]
/lib64/libc.so.6(clone+0x6d)[0x7fc5f7f06bdd]

Trying to get some variables.

Some pointers may be invalid and cause the dump to abort.

Query (0x7fbe2d9141f0): is an invalid pointer

Connection ID (thread ID): 8227134
Status: NOT_KILLED

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_push$

The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.
We think the query pointer is invalid, but we will try to print it anyway.

Query: ALTER TABLE `subject_bpztools_aagenda` ADD `id_subject_cat` int(11) NULL DEFAULT '0' AFTER `id_subject`, ADD INDEX `id_subject_cat` (`id_subject_cat`)