Ok, yesterday I came onto the site to check out the 'newest posts'. There was two pages of topics listed, and I usually like to read the oldest first, so I clicked on the 'next' (also tried the '2' later) and got this:
We've just had this problem on one of our servers and found that it is caused when the php session directory (specified in php.ini) cannot be written to by apache. In our case this was caused by the disk partition on which the directory was located had been filled up. I'd suggest that an other cause may be problems with permissions ...
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):