If you ever had problems with corrupt databases in an Exchange 2010 DAG setup you might have encountered this error. Trying to activate the database on a server where it has this state will fail.
However there is a quite simple fix given that you have a healthy copy of the database on a different server:
Update-MailboxDatabaseCopy -Identity DB<server with failed content index state> -SourceServer <server with healty copy> -CatalogOnly
Run the command in the Exchange Management Shell and it should fix the Content index state and allow you to activate the database again.