Are you on a TOR connection?
I experienced that yesterday when I tried to access the site on a TOR connection. 
I played just few minutes ago, completed a challenge, but did not receive the same message. I'm using a standard browser on a standard connection.

Just a reminder, they also have a raffle for players that will be able to complete at least 20 gift boxes out of 24, so if you are planning to partake, secure a box within 5 days from now.