I expect, that you have enough experience with MySQL/MariaDB to alter the column definition for mlf2_useronline.ip yourself from the length of 15 to 128 characters. Please report, if this changes anything.

YES!

The error has been found!

Sometimes it pays to stare at goats … errr … at code. 😆👍

So for everyone. Please run the following SQL query once.

ALTER TABLE mlf2_useronline
CHANGE `ip` `ip` VARCHAR(128) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT ''

Tschö, Auge