Can't connect to Server 5 via WebDAV

I'm trying to share out a folder to Mac OS clients via WebDAV using Server 5 (on OS X 10.11) and whenever a Mac OS client (10.11–10.13) tries to connect to the server, before they're even asked for login credentials, they get the following error:

There was a problem connecting to the server.

This file server will not allow any addition users to log on. Try to connect again later.

Yet, connecting to the same server/Share Point, via AFP or SMB...there are no issues. It's only when connecting via WebDAV.

I have a feeling this is a generic error and it's not actually an issue with there being too many users (as I get the same error when I do a reboot on the server and there are no users connected, via AFP or SMB).

Any else run into this?

Any solutions?

Thanks,

Kristin.