I’d be interested to find out what the cause is of this problem.
I also had the ‘a:1:{s:5:./././././././././././././././././././././tmp’ problem with the active_plugins field in the database – as well as the same problem with the directory specified for uploads.
Resetting these back to what they should be (just empty the active_plugins, and fix the file uploads path) managed to resolve my problem.
What is causing this? Some sort of bug / loophole in WordPress?