My server support has become very trying recently as the forum is still down and I already told him to do what he suggests below:
"The server does not have APC installed and it is not working with php 5.5 at this time. I'd advise in this situation downgrading back to php 5.4 to see if it resolves the issue again be advised there is little i can do at this time as we do not provide development support with the forum and its script operation. Let me know if you would like to have the recompile ran as this is the only option i can see at this time."
@Ogreman If you enable debug mode in your config.php file as Brogan recommends, it will show a more verbose error message. If you're using APC for caching, then that's probably it.
In PHP 5.5, Zend OPCache was merged into the core. It replaces APC as the standard opcode cache, but does not include data caching. A fork of APC that provides only data caching, called APCu, can be used with XenForo to retain this functionality while using Zend OPCache.