I have tried
hostname\adminuser
adminuser
domain\adminuser
and as I stated In my post I have logged on to the virtual center with each of these credential from the same browser instance.
when I attempt to register with bad credentials it responds with the correct error, but failed to register and no indication of why or what logs to check is not very helpful.
The homepage of the application does not provide log location to debug the failure, one of our network scanners could be the issue but need more data on the error/failure.
Thanks,
Paul