I fixed it by removing the “faulty” user I had added to the group. For me it was “faulty” in that I didn’t prefix its id with its connection name when I added it to the group using the extension’s API.
E.g. I added it as {some-uuid} instead of email|{some-uuid}. Removing user {some-uuid} from the group and adding it again as email|{some-uuid} fixed it.