[horde] Getting "invalid application" while trying to safe horde config

[Tom Bühlmann]

Hello again!
I decided to start all over with my installation.
I used the "horde-pear-uninstall"-Way described here:
http://wiki.horde.org/UninstallPEAR

Have horde fresh installed and running.

If i try to (for example) set my database type ($conf[sql][phptype]) the 
browser gets short black
and throws me back to the main configuration screen. in the right corner 
i see a red square telling me "invalid application".
Same happens if i try to safe my config in the browser.
  Neither the test.php nor the apache logs show something.
Any idea what could have happend.

Thanks for helping!
--tom