I had this problem in exactly the setting mentioned above. However, the porposed solution didn't work for me. In the end I found out, that it was not possible to call the URL for the portal web adaptor (https://[web server name]/portal) from the machine where ArcGIS Server was installed. But this is mandatory, since ArcGIS Server will call the /generateToken endpoint from portal through the web adaptor. So opening port 443 for communication between the web and the GIS server solved the issue.
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