Exchange Management Shell "Could not find any available Global Catalog in forest"

Resolved - this was a DNS problem.

Specifically, in my DNS forward lookup zone for my domain, the Name Servers, _msdcs node, _sites node, etc contained references to old or invalid Domain Controllers.

These references included:

• Domain Controllers which had been decommissioned (by demoting properly, I must add) were not removed from here.
• Domain Controllers which had been installed in one site but subsequently moved to another site retained references in both sites.

Attempts to connect to one of these Domain Controllers were causing Exchange02 to fail to find a GC.

Cleaning out the bad records, then rebooting the Exchange servers (I could have rebooted only Exchange02, but I decided to do both with Exchange01 used as a sanity check) resolved.

Because Exchange is a platform that you can't reboot too regularly (at least without it being service-affecting) I believe that this problem would have eventually manifested on Exchange01, and possibly even disappeared from Exchange02, given a sufficient number of reboots.