Lets try something else. As someone has suggested earlier on, force yourself to get the 500 error a few times, then immediately visit your error log and give it a post here.
Also, looks like ya broke something, I can't access your site
Code:
MyBB SQL Error
MyBB has experienced an internal SQL error and cannot continue.
SQL Error:
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') ORDER BY u.username ASC, s.time DESC' at line 4
Query:
SELECT u.avatar, u.username, u.uid FROM mybb_users u LEFT JOIN mybb_sessions s ON (s.uid=u.uid) WHERE s.time > '1421960369' AND u.usergroup IN () ORDER BY u.username ASC, s.time DESCSC, s.time DESC