I'm running out of ideas here, but have 2 other suggestions to try:
- Change the webdav url in Joplin to use /webdav/ instead of /dav/ in it. If you get an error initially, try clicking sync again anyway and see if it works. This was suggested here: "Unsupported WebDAV URL format" ERROR - #8 by dpoulton
- Finally, what about if you set chmod 777 on either or both of the joplin and changelog directories, and all their child files and folders recursively. A bit overkill, but worth a try