solved.
The generic user account that we use for URL OTA enrollment needed additional permissions.
Not sure why, but previously on the account, the only items checkmarked were for:
Computer Enrollment Invitations - CREATE
Mobile Devices - CREATE
Shooting in the dark, I gave all permissions to that account for
Computer Enrollment Invitations
Mobile Devices
Enrollment Profiles
That fixed it.
I probably did some overkill, but in the heat of the moment trying to enroll some important devices, I can work until I figure out which permission fixed it.
I ran into the same situation. If I go back to see what minimal rights are need I'll post here. At this point it's not high on the list though.
My guess is that you only need 1 extra permission turned on in mobile devices - i turned them all on, and it allowed me to log in. I wasn't able to experiment to find out which ones weren't necessary.