I have had 4 users (out of about 100) so far that cannot use the URL or the invite to install MDM on their iPhones.  They are all the same model as other users, their accesses are similar, nothing is different.

When they get to the AD AUTHENTICATION page they enter their email address, username, and password.  They click Continue - and all it does is cycle.

Anyone run into this and discover a fix?  Or is there anything we should be checking?