Can't edit articles since upgrade to 3.0.20 PL1

Upgraded to 3.0.20 PL1 yesterday and have just noticed that clicking on an article's "edit" tab (or edit link in article section) brings up a "blank" page. These are all articles that pre-date the upgrade. The pages are there but there is no content to edit? Same thing with "History" tab - there is no history shown. Actually, the only tabs that work are "Article" and "Discussions".

I reverted to the vbulletin default style and all was okay there so I created a new style using my usual style as the parent and everything is back to normal.