Rooms and Resources in Domino can be restricted to who can book them. This is configured in the Resource Reservation database.
When users in Outlook send a meeting request to Room or Resource in Domino which has this kind of restriction, even though they are the owner of the Resource the Insufficient access error is seen.
This issue is caused by the SMTP address not being populated in the owner field.
Edit the list of names that can book the resource replacing the Notes name with the SMTP address of the user, making sure the address matches the External SMTP in Exchange.
To allow other users to book the Resource, they will also have to be added to the Owner field thus negating the Owner restriction functionality in Domino.
This is by design in Domino as the field is set to only look up Notes style names.