As anyone encountered this error? I've recreated the VDI/RDS environment multiple time, each with the same result. Out of the 15 we are spinning up, 5 always fail. It's seeming random which 5 do not join (i.e.: It isn't the first 5 or last or in sequential order). In this dev subscription, we have 10E5 and 5E3 O365 licenses - this is the only clue I have; however, those licenses are all eligible for WDS.
UPDATE (Solved) Issue was with AAD DS domain join limit. Even though the user account was a "Global Admin", it was missing the permission "AAD DS Administrator". Incidentally, this is not the "Device Limit" in the devices blade of users. This is the " ms-DS-MachineAccountQuota" in ADUC, which MS hardcoded to 10 device joins. The "AAD DS Administrator" permission overrides that limit (obvi).