SOLVED using this:
It’s a known bug in Facebook API that breaks posting to groups using non-numeric names.
There is a simple workaround for it.
Please use the numeric URL of your group in the plugin settings, https://www.facebook.com/groups/{group_id}/
You can find your Group Id in your source of your Facebook Group page by searching for group_id=
You will need to Re-Authorize your account after you update the URL.
