Drupal doesn't save any changes in admin settings

I have a strange p开发者_运维问答roblem in drupal. When I'm trying to change _any_ setting in the drupal admin (caching under performance, temp-directory under file-system, default filter under filters, etc) I get the message that the changes were saved successful, but the values don't change.

I don't know where to start debugging since this is such a widespread problem. I've checked rights of all files/folder and the database connection. Seems fine.

Anyone experienced such a problem before?

edit: There isn't a single error entry in the drupal log file.

edit 2: I just deactivated every single contrib module. Still I can't make any changes to (for example) the caching mode.

Clear the cache in application and check again .. You can find it from

Drupal 7: Administration > Configuration > Development > Performance (admin/config/development/performance)
Drupal 6: Administer > Site configuration > Performance (admin/settings/performance)

Click “Clear cached data” button below