Hi Matthew! Sorry about the problem.
Before I get into that a bit, I checked, and your invitation was accepted before, so you should've been able to complete the challenge before, too, by clicking Submit and following the other instructions.
Our server that handled this was sleeping as it was on a tier with limited hours which were used-up due likely to some other higher-traffic. We bumped up the tier so it would run again, and it's working.
Thanks for reporting this, and let us know if things are working for you now!