I run in the same WebDAV sync problem an all my Enpass clients (maOS, iOS, iPadOS) after upgrading our WebDAV Server package on the Synology NAS.
I upgraded the WebDAV Server on my Synology NAS with DSM 6.2.4. from WebDAV Server Version 2.4.0 to 2.4.1. https://www.synology.com/en-global/releaseNote/WebDAVServer
Interesting: All other iOS WebDAV apps running without WebDAV issues ...