@ScleppelAug 02.2005 — #[B]BEFORE DOING THIS I WOULD SUGGEST BACKING UP YOUR DATABASE[/B], I take no responsibility for any problems doing the below may cause, i got it from the source below.
Apparently it can be fixed by going into PHPMyAdmin (or your MySQL admin program) going to the PHPBB database and typing [CODE] REPAIR TABLE phpbb_sessions [/CODE] into the SQL textbox.