When attempting to log onto a Data Domain's management webpage, sometimes you may run into the issue where you are not able to log on, and the following error appears, even though you have not upgraded the firmware or there's nothing wrong with installed SSL certificates.

Service Not Available
The GUI Service is temporarily unavailable. Please refresh your browser. If the problem persists, please contact DataDomain support for assistance.

While the webpage does not work, it is still possible to log on using CLI (through SSH).

If you have not upgraded DDOS, try restarting the web services by logging on through CLI and running the following commands in sequence:

adminaccess disable httpadm  ...