There is a bug in Exchange 2010 SP2 that has yet to be addressed in any of the Update Rollups, including Update Rollup 5v2. When attempting to move an active database copy to another mailbox server, the following error can be produced for no reason whatsoever. All passive database copies can be in perfect health with copy and replay queue lengths at zero and this error can still occur.
All that is needed to resolve this is to suspend the database copy on the server you wish to activate the database on, resume the copy and then move your active database. In our case, we would suspend DB3 on MBX-01, wait for it to reach its suspended state, then resume it. Once it is healthy again, attempt your active database move and it will succeed.
Microsoft, please fix this.