When rearranging and upgrading to never OS it’s very common to install a new DC and then remove the old one instead of doing in place upgrade. This is all fine Active Directory wise. Exchange Server itself will dynamically detect new and removed Domain Controllers and Global Catalog servers and adjust accordingly but Exchange Management Console is not dynamic in the same way.
You might also see error messages in the Application log indicating unavailable LDAP or DC/GC.
Why is this happening?
EMC caches Domain Controller names in the MMC temporary files and unfortunately it will not be clever enough to query another DC or GC when the current one is not responding or has been removed.
How to solve the problem?
Solution is quite easy: clear the MMC cache.
Close all open MMC. Start a new empty MMC, in the menu, select File/Options and then on the Options Window and Disk Cleanup tab, press the “Delete Files” button. Close the empty MMC.
Next time you start EMC it has no cache to read from and will dynamically select a new DC.
There is another way of doing this. Close all open MMC and then delete the file “C:\Users\%username%\AppData\Roaming\Microsoft\MMC\Exchange Management Console”.
As you can see this cache is per user which unless the administrator creates a process of clearing the cache when needed, each user that runs EMC must clear the cache itself.
If you’re interested, “C:\Users\%username%\AppData\Roaming\Microsoft\MMC\Exchange Management Console” file is in XML format and can be viewed if you like, but I wouldn’t advice you to make any changes to it.