The error code indicates that the database that stores login session ID's has become corrupted. That database normally is stored in memory and backed up to drive regularly. The error implies that both the one in memory and the one on disk are corrupted, and that the PHPBBS self-fix routines have failed.
Odds are nothing important is lost, but until it's recreated or repaired, it can't get valid lookups to see if someone is logged in.