Receiving a service unavailable response on a website usually indicates, that the application pool of the site is not running.
To solve this issue, open your InternetInformation Management and verify all required app pools are started.
The page content would look like this:
HTTP Error 503. The service is unavailable.
Embedded in CRM the error would look like
Figure 1: Service Unavailable error message
The issue can be caused by problems during the upgrade, because upgrading stops the app pool in order to avoid issues with files in use.
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