Hosted Exchange - Modify MailboxPermissions: "it's out of the current user's write scope."

Hello,

if I try to modify the mailbox permissions ("Add-MailboxPermission -Identity <> -User <> -AccessRights <>" or remove it) I get the following error message: 

The operation on mailbox "Mailbox" failed because it's out of the current user's write scope.

I tried a lot of different things like changing the role groups or end the cmdlet with the parameter -IgnoreDefaultScope and so on. Is there a way to get "super administrator permissions" to modify all organzionalunits?

I spent a lot of time searching for this error message, but I couln't solve the problem at all. I think I will need a real specialists :)