If this question has been asked before, I apologize, but there is no archive of past questions, so...

Whenever I'm posting a thread and I go inactive for a bit, whenever I post it it logs me out and then once I log in it tells me that the thread I was posting in was invalid o_O It's a very odd glitch that I've never seen before, but it has a tendency to log me out about every 15 minutes if I don't click on a whole bunch of stuff.
Anybody else have that problem, and even better, is there any way to fix it?