Theme interference with WP backend

Set up my blog under WP 2.2.1 with a custom made theme a year ago. Not a single bug during many months – although my site has a wide variety of features (plugins) and regular postings.
Several weeks ago I receivied a WP message “WordPress not installed” when visiting my site. Checking my database, I found the all tables were crashed. I was able to do a repair – but since then my theme interferes with my wp backend.

The symptoms are:
– Cannot edit pages any longer. The Edit-Page is covered with lines of code from different templates of my theme. Save-Button is visible, but not clickable. I CAN edit posts without any problems.
– Cannot edit my theme files in WP. Same problem on Edit Theme Page.
– Admin > Plugins shows the complete code of the plugins where only the title should be. Activation/Deactivation is possible.
– 2 Plugins (Anarchy Media Player, Simple-Tagging) generate Syntax Errors, although I did not change a line, so I cannot activate them
– Admin > Manage > Shoutbox (from Rudd-O’s Shoutbox Plugin) generates a “No access for You” message

Completely deleted database and site and rebuilt them from backups of 2007 when everything still was working. No difference.

Tests:
– Switching to default theme and even deleting my custom theme
re-enables editing pages and themes. but mess on plugins page stays even after deletion of my theme!

My theme is very nice and it took me months to create. I can’t understand why it now interferes with WP’s backend after working fine for nearly a year, when I did not change a single line of code?

I really do need help. Is tere any workaround or solution to this?

Please, help,
Peter