This is similar to messages observed during OLCI on UA when you are going to be subjected to the Super Secret Security Screening (boarding pass shows SSSS on it once you get it, but you can only get it in person).
Thanks for the heads-up. I guess maybe my family and I have a new experience waiting for us in the morning.
Considering Frontier's web-site says our luggage has to be checked in one hour prior to departure, I think your recommendation of an extra 30 minutes makes a lot of sense. I'm just not interested in providing Frontier an extra 120 minutes of my time when they can't even tell me what the original error message means.