After upgrading to 5.8.x, you may have noticed that you are no longer to access the PMAdmin site despite having the domain account added to the PM server’s local PMAdmin group.
This is by design, as PM 5.8.x has introduced a new authentication method to the PMAdmin site. A documentation defect (TF00170441) has been raised to address it.
From 5.8 onwards the Users need to be part of either "IIS_IUSRS" or local "Administrators" Group, along with local "PMAdmin" Group, to be able to access the PMAdmin site.
The product documentation will be addressed in the forthcoming release: 5.9.