Black screen of death is actually: Gateway: User name is missing. Twice in two days - but why isn't there a 'home' page which just tells us straight away which parts of the service we have paid for are not currently working. With an ETA - is there such a page?
Where are you receiving this message? On login to Hosted itself or when attempting to launch the Hosted 2018 service?
Which browser are you using? Do you receive the same message if you attempt to use Hosted on an alternative browser?
Can you please check if there are any active sessions listed in the Hosted control panel. If there are, end them by clicking the 'Log off user' option in the respective session then logout, ensure that you use the logout option in the top right corner.
Close your browser and then try again after 3-5 minutes.