I just had the same error yesterday
I wasn't working on my forum and my host didn't make any changes to my domain settings or whatever..
that error just pop up out of nowhere..
In my particular case it was lacking space in the tmp folder. The tmp folder I was using had been filled by another client of my web hosting company (it's a shared tmp folder).. and it triggered the error on my forums.
I guess I'm just saying that in some cases you just can't fix this problem if you're not the server admin.. I spent the whole nigth yesterday trying to figure out a way to 1-find this tmp folder and 2-clean it. Little that I knew that there was no way for me to fix that anyway due to my own settings. So yeah, you may want to speak with your webhosting company if this kind of error happens.
(bumped for additional infos purposes..)