Have reviewed your install and it's due to the Forum of the Category the Group belongs to not being Public accessible. However, you couldn't see or change this parameter as forum categories were set to be hidden within GJ > Config > Integrations > Forums. I toggled category forums, set the categories forum to allow for public, then toggle category forums back off. The issue appears to be resolved for the test group you provided. Please also confirm is working fine. If you run into the issue again please follow the below steps.
1. Navigate to GJ > Config > Integrations > Forum > General
2. Set "Categories" to "Show"
3. Navigate to GJ > Categories
4. Edit the category having issues
5. Set "Public" to "Yes" under the "Forum" integration tab
6. Click Save
7. Navigate to GJ > Config > Integrations > Forum > General
8. Set "Categories" to "Hide"
Will review fixing this for next release as those parameters should be accessible from backend still regardless.