Found an answer on the vbulletin forums. Apparently a corrupted cookie can cause this, so clear all paradox cookies and everything should work as usual again. I have firefox in Swedish for some reason, but the path you be something like this: Tools - Settings and under the button with the theatre mask on it you'll find "remove certain cookies". You probably don't want to remove all cookies, so just search for paradox or morboth and remove all of those.