Room Mailbox and Auto"reject"

Hi guys,

i have an Exchange 2013 (migrated from 2007 i guess..) and a few room mailboxes.

So basically if i made a calendar meeting with the room mailbox and there is also another meeting, it gets the state "reservation" and not reject automatically.

AllowConflicts ist False, so i would expect, that this meeting will be accepted or reserved.

Do you have an idea why it gets automatically rejected and not reserved?!