"You are not permitted to view the requested resource" during Invite dialog flow

Hi.
I’ve searched and couldn’t find anything about this. We have all of our staff users (except admins) unable to invite to the forum.

The invite dialog gets a pink error at the top whenever either of the submit buttons are pressed.

“An error occurred: You are not permitted to view the requested resource.”

The only discussion I’ve seen of similarly-worded errors revolved around API keys. Our site only has one enabled (for email replies), and it is configured for “All Users” already.

Does anyone know how to get around this? Seems to have started in one of the updates over the past 6 months.

I’m on the latest stable release.

Thanks for any info!