Last week, my colleague tried to unregistered plug-in using Plugin Registration Tool and got above error message.Since it was very generic error message, I tried to look detail error message in Event Viewer but have no luck to find it.
One of my colleagues suggested me to check whether log-in account had deployment manager role or not. I tried to open Deployment Manager MMC and got following error message.
Below is the detail error message I got from CRM trace log.
System.DirectoryServices.ActiveDirectory.ActiveDirectoryOperationException: Unable to obtain DNS hostname of Active Directory domain controller with ntdsa object name "CN=NTDS Settings, CN=[DOMAIN CONTROLLER], CN=Servers, CN=Default-First-Site-Name, CN=Sites, CN=Configuration, DC=[CRM SERVER], DC=[DOMAIN NAME], DC=org, DC=sg".
Resolution:When we unregistered a plug-in, CRM tried to query login user credential and role from AD. But Active Domain Controller information is pointed to Replicated AD which was no longer available at that moment. Then, plug-in registration process had assess right issue and we got "Server was unable to process request" error message.
Remove unused Domain Controller from "Active Directory Sites and Services" will help your problem. Please refer to Reference link for more information.
Reference:Delete Failed DCs from Active Directory
Delete Failed DCs from AD.pdf