Problem solved. The error at the top of the page was caused by the Simple Tagging plugin. Once I deactivated that everything worked fine. Now I just have to go back and re-tag a bunch of articles.
I finally found some time to update my wordpress from 2.1.3 to the current version. Things are a bit bumpy right now as you may be able to see the error across the top of the page. All of the articles are still there and can be reached via the archives and categories. My tags [...]