The hosting platform returned an error message while trying to process your request. Please try again or try making the change through the control panel. The error returned is: Invalid Domain, please check your input. .9
I don’t know exactly why this was setup as a restriction on the hosting platform, but I know that it was added there deliberately.
This particular issue was that the check in the client area was more specific than the one in the hosting platform, so it only triggered when the account was being created, instead of during the domain validation step.