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!
There was lots of useful information in the issues for DavDroid at GitHub.com which I cannot locate now that the code is no longer hosted there. Is there an archive or a import of those issues available anywhere?
They have been imported into this forum.
And I have added a Solr server yesterday to make the NodeBB search more useful