Thanks for this. It seems you have MySQL on some kind of stricter mode than usual, since in most cases this should just generate a silent warning. But it is a bug, which you can fix by changing line 193 of qa-app-users.php from:
qa_db_user_set($db, $_SESSION['qa_session_userid'], 'sessioncode', NULL);
... to ...
qa_db_user_set($db, $_SESSION['qa_session_userid'], 'sessioncode', '');
Of course I'll also fix this for the final 1.0 release.