I updated my test forum from phpBB 3.0.10 with MODs to phpBB 3.1.0 a2, found one of the MODs (Auto Backup) as extension, so I tried it on newly updated forum. When I enabled the extension and went to it's settings (in ACP - Maintenance tab - Auto Backup) I got this error:

General Error
Cannot find module ./../includes/acp/acp_auto_backup.php

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/includes/functions_module.php
LINE: 506
CALL: trigger_error()

FILE: [ROOT]/adm/index.php
LINE: 71
CALL: p_master->load_active()

When I disable the extension, delete it's data and enable it again, it works.

I wrote about this issue to author's extension topic and was told to create ticket here.