I had the same problem. In my case, the reason was that the default post category (as defined in the WP Settings - General page) was set to be locked. The way I fixed it was to recreate a new category (call it anything for now), set this as the default post category in WP Settings - General. Then delete the old default category using the option to transfer all the related posts to the new category you recreated. You can then rename this new category back to what you had originally.
By doing this it seemed to free up the errors that had been created during the upgrade. My site all now works exactly as described.
Hope this works for you too... James :)