If you are experiencing WebDAV issues with database sync and you are using Synology NAS, there are a few steps that you can try and fix the problem:
In Synology, go to System Settings > Security > Extended and check the "Intermediate Compatibility" option.
In the pop-up window, in the WebDAV Server line, select the "Old compatibility" option.
After that, just hit "OK" and "Save" and your issue should be resolved.
There are more helpful tips and tricks available here:
NOTE: Synology DS File doesn't work very well with with iOS "Files", so we always recommend that you use WebDAV.