Hi Hans, Your community is ...
Published by Elena Steiner (retired account), Employee at New company to test API
Hi Hans,
Your community is set to private and registrations are only allowed if created by a manager though the Manager Panel. I would guess that this issues is related to sending an invitation from the member list rather than the Manger Panel. Can you please check if you have manager permissions and send the invitation from the offical Add Member option found in the Member Management section of the Manager Panel.
More details on how to add member from the Manager Panel can be found in the tutorial Member Mangement