When I try to pick a member to send a message to this is what pops up on the screen (not in the PM part but where it list the members or if I click on a name to send a message to):

mySQL query error: SELECT t.forum_id, COUNT(p.author_id) as f_posts
FROM ibf_posts p
LEFT JOIN ibf_topics t ON ( p.topic_id=t.tid AND t.forum_id IN (0,35,32,34,33,5,8,7,31,30,1,2,6,4,12,13,14,15,16, 28,11,27,18,19,20,26,41,40,3,29,9,10,25,21,22,23,2 4,36,38,39) )
WHERE p.author_id=1087 AND t.tid IS NOT NULL
GROUP BY t.forum_id
ORDER BY f_posts DESC

SQL error: Can't create/write to file '/tmp/#sql2121_160fe_0.MYI' (Errcode: 13)
SQL error code:

I don't know if that helps or not.