I have the same problem using the webdav from 1&1 provider in Germany. Everything was working just fine until two or three updates back. I do not think it has something to do with the one or another webdav provider but with Joplin.
I tried your suggestion with syncing simultaneously on both devices but it didn't help. I am using another webdav provider, 1&1 from Germany. Everything was working perfectly until few months back