::Update::
The normal template configuration can not be changed anymore, too. See screenshot, the "Message:
Updating css for the onepage2 theme." stands still, nothing happens. Only way out is to relaod the site, then every change is gone, nothing saved.
Site is still on development, nothing changed in the settings on the server. Could it be a problemm with the rights in writing? Have no idea why this should have changed but could it be a possibility for this?