[[!comment format=mdwn
 username="sts"
 subject="comment 1"
 date="2015-12-07T08:12:26Z"
 content="""
ok, I could find the source of the problem: they use sabredav as WebDAV server and sabredav does not support chunked transfers:

        // Intercepting the Finder problem
        if (($expected = $request->getHeader('X-Expected-Entity-Length')) && $expected > 0) {

            /*
            Many webservers will not cooperate well with Finder PUT requests,
            because it uses 'Chunked' transfer encoding for the request body.

            The symptom of this problem is that Finder sends files to the
            server, but they arrive as 0-length files in PHP.

            If we don't do anything, the user might think they are uploading
            files successfully, but they end up empty on the server. Instead,
            we throw back an error if we detect this.

            The reason Finder uses Chunked, is because it thinks the files
            might change as it's being uploaded, and therefore the
            Content-Length can vary.

            Instead it sends the X-Expected-Entity-Length header with the size
            of the file at the very start of the request. If this header is set,
            but we don't get a request body we will fail the request to
            protect the end-user.
            */

            // Only reading first byte
            $firstByte = fread($body, 1);
            if (strlen($firstByte) !== 1) {
                throw new Exception\Forbidden('This server is not compatible with OS/X finder. Consider using a different WebDAV client or webserver.');
            }
          
            ...

Although, I did not told git-annex to chunk the transfer :-/, because I did not append a 'chunk' parameter. Any ideas how to fix that?
"""]]