XF 1.5 Permissions Error

I keep getting 'You do not have permission to view this page or perform this action.'
I can see the forum fine when I'm not logged in but as soon as I log in i get the permission error. I've double and triple checked all my User Group Permissions and Node Permissions and they are turned on.

The error I keep seeing in my logs is:

ErrorException: Fatal Error: Class name must be a valid object or a string - library/XenForo/Model/Node.php:521
Generated By: Unknown Account, 1 minute ago
Stack Trace
#0 [internal function]: XenForo_Application::handleFatalError()
#1 {main}
Request State
array(3) {
["url"] => string(31) "http://forum.dvunderground.com/"
["_GET"] => array(0) {
["_POST"] => array(0) {


Well-known member
I get this error when I try to access the forum :

"forum.dvunderground.com is currently unable to handle this request."