I recently had an issue with a Windows Server 2012 machine where if I ever tried to add or remove a feature I would receive the following error:
Failed to open the runspace pool. The Server Manager WinRM plug-in might be corrupted or missing.
After a number of hours searching on and off, I found a somewhat related solution to the problem here. The first step is to run the following command to see what addresses you are listening on:
netsh http show iplist
I found that my machine was only listening on 192.168.0.2 (its main IP address). So, based on the solution I found, I added the loopback address (127.0.0.1) using the following command:
netsh http add iplisten 127.0.0.1
And voilà, the error disappears and everything works as it should.