@daikin Maybe it's just a slightly confused error message, meeting rooms needed the same mechanism to allow Whiteboard access as external users.
Whiteboards are now a OneDrive file and use the normal sharing approach of OneDrive to make that available to all the users on meeting request. Meeting rooms themselves are different, they may or may not be invited to the meeting (e.g. you make have user proximity join to add the room). You also don't really want the rooms account to have ongoing access to the whiteboard after the meeting.
To resolve this the Whiteboard team developed an alternative sharing mechanism where rooms and external parties are granted edit access to a whiteboard temporarily during a meeting, and that then stops after the meeting is complete. This is separate and in addition to the standard OneDrive permissions.
The end result is that external and meeting rooms use the same, alternative method to get permissions to a whiteboard just while the meeting is running.
