This isn’t a huge issue, but it would be nice if there was a note there saying what is wrong. I unknowingly encountered the error and didn’t know what was wrong until I noticed I had no group owners.
I had a quick look at this. The form is posted and the generic error is coming from the back-end when validations fail there.
Adding front-end validation and returning a descriptive error message from the back-end both seem like reasonable fixes.
Adding on to this, when you make the group private further down the form, these checkboxes get disabled, but if you already checked them they will remain checked. This renders the front-end guards are ineffective, and lets you create a private group with an access policy requiring public visibility.
Edit: The front-end validation might not be so straight-forward, because the form is split so that the dependent parts are in different components. Once you create the group, they are in separate tabs, which is I guess not ideal. Adding on to this it’s also a legacy Ember component with two-way bindings.
You can also remove all owners after creating the group, which renders it defunct.