Activity monitor is unable to execute queries against server

Solution 1:

Check if SQLServer performance counters exist in the Performance Monitor. If not, manually rebuild all performance counters running lodctr /R command with administrative priviledges:

cd \windows\system32
lodctr /R

Solution 2:

OK, I think I have found a solution.

The issue seems to be with the mix of 32bit and 64bit apps that need to query each-other.

If you enable the service Performance Counter DLL Host in the Services control panel, the Activity Monitor should now work.

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Solution 3:

In the simplest case all you need to do is to restart the SSMS. I just had this problem with SSMS 2008 R2 running against 2005 server after I had lost network connection while the Activity Monitor was running. I tried a couple of tricks before I decided to try restarting SSMS and that's what helped.