There has been a decent amount of chatter externally about this, so I was very glad that we finally got this article written:
932599 You can no longer mount any of the Exchange databases after you apply hotfix 926666 to a computer that is running Exchange Server 2003
Since this issue has to do with permissions that are added to the Organization level and to the Administrative Group level, I recommend that you test the install of the 926666 patch on one machine in your Organization prior to rolling it out across all machines. After that you can test on one machine in the other AGs as well. If no problems occur after that testing, you will probably be safe from this issue on the other servers as well.
There are two possible reasons for this happening: BUILTIN accounts incorrectly added to permissions or duplicate accounts showing up. We have workarounds currently to resolve both of these issues as well as a hotfix in the works (for the duplicate accounts) that is not yet released. The issue with the BUILTIN accounts being added to the permissions is “by design” and will require you to follow the workaround.
You will know you have this issue if you see the following events after the databases don't mount:
Event Source: MSExchangeIS Event ID: 9519 Description: Error 0x89a ...
Event Source: MSExchangeIS Event ID: 9518 Description: Error 0x89a ...