I am running an all-in-one Exchange 2010 SP1 (version 14.1 Build 218.15) on Windows Server 2008 R2 SP1.
I have had the perfmon counter MSExchangeIS/User Count set up and have been able to view the values for users connected to Exchange. Recently, the value is "1" and is stuck there although users are able to use the system etc.
I have read elsewhere that a reboot will reset the situation but the issue may recur.
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