I solved the problem: some owncloud app added an invalid character in the xml answer to the client request. now it works great, thanks for your efforts!
I want to sync with Horde. While creating an account (base URL mode) I get an error:
Missing capabilities: Invalid DAV response: 207 Multi-Status expected but not received.
Fix: I found out that if I don’t use my email address as username but the postbox username I could proceed and sync.
Remark: If I enter the base URL in a browser I could successfully login in with my email address and see the SabreDAV webinterface.
Ok, thanks for the info!