Looks like it's time to migrate to a more stable framework... :(.
Update: there were some errors in app.cfg. Everything seems to be working for the moment. I guess I can afford to put off the migration for a while longer.
After an apt-get dist-upgrade on Debian unstable, TurboGears no longer works - it depends on python-turbojson >=1.1.4, <1.2. The Version in Debian is now (you've guessed it) 1.2. I've downgraded turbojson to 1.1.2. Now, styles no longer work.
Looks like it's time to migrate to a more stable framework... :(.
Update: there were some errors in app.cfg. Everything seems to be working for the moment. I guess I can afford to put off the migration for a while longer.