Some users have had issues determining their WebDAV URL when they install Nextcloud as a Snap, on services like DigitalOcean.
The correct URL format looks something like so, where “mark” is your username/home:
https://example.com/remote.php/dav/files/mark
The following GitHub issue goes into much more detail:
https://github.com/nextcloud/nextcloud-snap/issues/514#issuecomment-382096885