I had exactly the same problem yesterday (2020-12-18) - black screen after "The onboarding process will..." message window. I have tried different network settings, other PC, lowering screen resolution... nothing solved an issue. About three hours before the exam and also few days ago, everything worked fine and successfully went through hardware compatibility test.
I couldn't take exam on 2020-12-18 because of the "Black screen" - very frustrating, then, the day after I could login and successfully perform the hardware compatibility test (this was yesterday and I thought it's all restored back to normal), and today (as right now) I can't get through to the login screen again - the black screen happens just like before. Definitely, the problem is not solved yet, and I still don't have any feedback on my support request.
It would be nice to see some official announcement...
I agree with you, at least they should comme with an official announcement. I have also send yesterday a message to support, I hope they will give me a feedback till tomorrow morning, if not I'll have to cancel my exam for 22 decembre