Since iOS 11 I do have problems syncing against WebDAV. Also I got the problem with the macOS version newer than El Capitan. My problem seems to be located inside the WebDAV implementation of iOS / macOS, as I do have WebDAV problems also if I use the macOS finder. The connection works only very few times of many. I opened a call at Apple but they do not offer a solution. Also i reported the problem at Synology, my primary WebDAV device. They know of the webdav problem of macOS but couldn't find a solution. Because the problem occurs also on an Nextcloud installation, I don't think it has something to do with Synology. From my point of view I like to ask you, if you could implement an own WebDAV client instead of using the Apple library functions, because otherwise there are only public cloud services left for synchronisation. If you need a webdav test-installation I could provide you with an account to my nextcloud installation in the internet. Update - I just tested the connection on an Nextcloud installation again and it seems to work. Maybe other users also use a Synology webdav installation.