For some reason the Mozilla network code we use percent-encodes periods in usernames. If I'm reading it right, RFC 3986 says not to do that, though it also says that servers should handle it (and most apparently do, or this would be a more common problem).
In any case, the latest Zotero beta should fix this behavior, so you can install that and let us know if it works for you, and then switch back to 5.0.55 once it comes out.
The WebDav server I'm using is Yandex Disk. I downloaded Zotero beta but it seems that it still cannot sync. However, when I downgraded Zotero to version 4, I can still sync successfully.
I've found the problem, it seems that Zotero cannot put a .prop file into the Webdav server of Yandex now. When I'm using Zotero 4 to sync, it succeeded in downloading the files I've uploaded to the server, but it fails to upload. And when I check the files it uploaded, it seems that the zip file has transmitted successfully while the prop file is missing from server.
Upgrade Storage
In any case, the latest Zotero beta should fix this behavior, so you can install that and let us know if it works for you, and then switch back to 5.0.55 once it comes out.