Facebook currently has a bug that is causing some posts posted by personal profiles to either fail or be posted improperly (showing an empty post or missing the picture or message) and also not allowing you to edit it on Facebook either. Facebook is working on a solution and you can follow our status page to receive updates about it. Click here for the status page.
In the meantime, the easiest fix would be to link the group to a FB page and schedule it to post as the page instead. Just so you know a business page group can still stay completely private, it won't need to be recreated, and your party guests will likely not even notice the change. You can click here to learn more about how business page groups work.
Then what you need to do is delete the party in the system from your party list using the red delete button in the Actions column. Go back to the Parties tab and select Schedule New - on that first step select Business Page Group and then you can schedule it that way - make sure to select the same start date as the original - it will skip anything already posted and pick up in real-time with the next scheduled post.
If you have any questions or run into any problems please reach out to support.