After upgrading to 1.3.5 from 1.3.4 I can no longer logon to the forums. The forum is visible but cannot logon. When trying to logon via the forums I get "A server error occurred. Please try again later." When trying to logon to the admin panel I just get redirected back to the admin logon again. If I try to run the upgrade script I get: Server Error Invalid authentication module '' (XenForo_Authentication_Core12) specified XenForo_Authentication_Abstract::create() in XenForo/Model/User.php at line 1251 XenForo_Model_User->getUserAuthenticationObjectByUserId() in XenForo/Model/User.php at line 1279 XenForo_Model_User->validateAuthentication() in XenForo/Install/Controller/Upgrade.php at line 296 XenForo_Install_Controller_Upgrade->actionLogin() in XenForo/FrontController.php at line 347 XenForo_FrontController->dispatch() in XenForo/FrontController.php at line 134 XenForo_FrontController->run() in /var/www/vhosts/domainname.com/httpdocs/forums/install/index.php at line 18\ I have tried to upload the files again and Im now Im trying to dowmload using another browser and upload again to see if that fixes this. Any help on how to resolve this issue please as the forum is basically dead at the moment.