Hi there,
I used the LetterPress template to build a website, and when I updated joomla to 3.1.1, something went wrong.

My website will function correctly, until I try to login to the administrator... then only the homepage will appear but any other page on the website will yield a page with this error:
Fatal error: JUser::getParam(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Joomla\Registry\Registry" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/lanarkstew/public_html/lscjoomla/libraries/joomla/user/user.php on line 286

A VERY temporary simple fix is to logout of the administrator, then login to phpmyadmin and delete all the sessions from the sessions table. But when I go to login to the administrator again (and a session is created, presumably) the error comes back.

Any help would be appreciated. Many thanks!