Since the beginning of 2017 Apple enforces the use of encrypted connections in all apps. To follow this guideline, we had to restrict the use of WebDAV connections that start with "http://". Even on your local network.
Please check your WebDAV server settings or ask your administrator to enable HTTPS. After you have enabled HTTPS on your server, you simply need to change the "http://" in your address to "https://". After that everything should work as before.