Private Messages database problem in 3.8.4

[AramisErak]

Had an error suddenly start showing up in the last few days.

Database error in vBulletin 3.8.4:

Invalid SQL:

UPDATE coti_user
SET pmtotal = pmtotal + 1, pmunread = pmunread + 1,
pmpopup =
CASE
WHEN userid IN(168) THEN 2
ELSE pmpopup
END

WHERE userid IN(168);

MySQL Error : MySQL server has gone away
Error Number : 2006
Request Date : Monday, January 27th 2014 @ 08:03:36 PM
Error Date : Monday, January 27th 2014 @ 08:04:36 PM
Script : http://www.travellerrpg.com/CotI/Dis...insertpm&pmid=
Referrer : http://www.travellerrpg.com/CotI/Dis...do=newpm&u=295
IP Address : 72.35.122.228
Username : aramis
Classname : vB_Database
MySQL Version :

I need help understanding the message.

Notes:
Occurs on sending PM's.
PMs still going through.
Sent PMs not showing in sent messages folder.