I was googling around a bit, and it looks like that is caused by a process limit. The Apache server is trying to launch more processes than allowed by the system.
I found
this pertaining to FreeBSD, but I'm guessing it's something similar for other operating system.
It looks like the Apache is culprit in this case, not the UBB.
*Edit* Guess great minds think alike, Charles beat me to the post <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" />
