I'd guess by the error message that this is could be helpful:
(01-24-2014, 09:25 AM)Error Wrote: Judging by your join date I would presume that what you're seeing is the infamous "1054 - Unknown column 'Guest' in 'where clause'" MySQL bug that we got after the IPB -> MyBB transition in late 2012. If that is the case, you can fix it yourself by following the instructions in the post quoted below.
(02-16-2013, 02:08 AM)Error Wrote: Have a look at this thread, and particularily my last post in it.
EDIT:
What you need to do is:
1. Download your old messages, if you want to keep them
2. Empty all of your PM folders.