It seems, i have found the problem. All my *.js files where owned by a other user on file-system level, so the www-data user becomes access denied:

<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
<s:sabredav-version>3.2.0</s:sabredav-version>
<s:exception>ErrorException</s:exception>
<s:message>file_put_contents(/local/path/to/client.js): failed to open stream: Permission denied</s:message>
</d:error>

After correcting the ownership, the upload seems to work.

Show us a verbose log files from WinSCP as well as from the other clients.

If i use WinSCP via WebDAV on my server and it try to upload a *.js file or *.json file, i get a http server error 500. Uploading with other programs like TotalCommander and WebDAV Plugin do work.