If I might chime in here, I believe we are having a similar issue. Ours stems from needing to run Apple Configurator 2 on existing iPads that currently aren't in ABM. When running through the configurator, it asks for the enrollment URL of the MDM solution (in this case, ESET Mobile Device Manager).
When using the enrollment URL for the new device (https://EMDM.xxx.xxx:9980/ebcsw5n7z), we're met with:
Unable to verify the server's enrollment URL.
The question is, is there a different enrollment URL we can use when enrolling unassociated iOS devices with ABM?