I check mysqld.log and there is note, that user who run backup was failed connection:
2019-11-24 0:45:12 21 [Warning] Aborted connection 21 to db: 'unconnected' user: 'mybackup' host: 'localhost' (Got an error reading communication packets)

This error occurred for every ONLINE backup run

Hi,
please do you have any idea?

Application and db is working ok but ONLINE mariabackup is still failing.
Is my DB consistent? I hope that yes. We don't set any encryption... There is output from my.cnf:

[mysqld]

1. 1. 1. General Section ###
         datadir = /poctt/001/mydata
         socket = /usr/local/dba/tmp/.mysql.POCTT.sock

sql-mode = TRADITIONAL
open_files_limit = 16384

user = mypoctt
port = 3306

1. 1. 1. Security Section ###
         local_infile = 0

1. 1. 1. Binary logs Section ###
         log-bin = /poctt/binlog/mydata/mariadb-bin
         max_binlog_size = 52428800
         sync_binlog = 1

1. 1. 1. Slow Query Log Section ###
         slow_query_log = 1
         long_query_time = 3.0
         slow_query_log_file = /poctt/log/mypoctt/mysql-slow-queries.log

1. 1. 1. Buffer Section ###
         key_buffer_size = 64K
         query_cache_type = 1

1. 1. 1. InnoDB Section ###
         innodb_buffer_pool_size = 2048M

1. 1. 1. 1. 1. ##### MySQL Server Startup Script #####

log_error = /poctt/log/mypoctt/mysqld.log
pid-file = /usr/local/dba/tmp/.mysqld.POCTT.pid

[client]
socket = /usr/local/dba/tmp/.mysql.POCTT.sock

[mariabackup]
user=mybackup

Thanks a lot Jan

Hello,
my problem is still not resolve. I found the same problem on another environment. I've two instances on one server and for the first instance ONLINE backup is working, but for second instance ONLINE backup is failing with the same error. On second instance ONLINE backup worked OK till date when I dropped databases and recreated again and made new import data. After this ONLINE backup has been failing.

Please I'm ready to sent all logs, config files and etc...

Can you help me, please?

Regards Jan