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!
Every time I change a contact I get a sync error.
It seems that DavDroid is not creating a valid VCard anymore (all the contacts were synced with an old version without a problem)
I attached links to three examples, as Im using the service of my mail provider I cant say if its related to a change on the server side, other tools like SoGo Connector are still working so.
(Actually I cant see a misstake in the Vcards sent by DavDroid but I am also not skilled in reading them…)