Error 0xfffffde0 starting database "First Storage Group\Mailbox Store (SERVER)" on the Microsoft Exchange Information Store.
This error is logged by the information store when it cannot mount a particular store, such as the private information store.
This can happen after a restore (see event 619) or when a defragmentation corrupts one of your Exchange databases.
See the article below for more information on how to proceed. As always, remember what happened prior to the store being unavailable and use that as base for troubleshooting.