https://mariadb.com/kb/en/mariadb/server-system-variables/#back_log

besides that the value is completly ignored - from where comes the 110 when the docs say it would be the same as "max_connections" which is on 300?

max_connections  = 300

back_log  = 1000

MariaDB [(none)]> show GLOBAL variables like 'back_log';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| back_log      | 110   |

+---------------+-------+

1 row in set (0.00 sec)

Default Value:

The lower of 150 or the value of max_connections (>= MariaDB 10.0.8)
50 (<= MariaDB 10.0.8)