Happens often in last time…
Do you get this error when you attempt to make a post on the forums? (new thread or reply?)
I sometimes get the error on the forums at the top of the page too, usually when viewing particular forum sections or threads, ie. forumdisplay.php or showthread.php
Today, the error message was nearly large as my display 
The forum doesn’t seem to like the new backend it’s on, it’s been a lot slower as well. I’m working to change it, but need to move several other things before I can make the forum happy again.
Just finished the first stage of improvements and I can tell the forum is a lot happier in it’s current state, so this should not occur anymore. 
I’m still getting the error.
For me, it’s another error