The certificate presented does not work for stackpromos.com, leading to a warning in my browser about an invalid certificate.
Word on the street is stackpromos.com probably isn't supposed to have a valid cert, but I linked it in the blog post as https out of habit. Will fix as soon as I can get the blog post edited!
We now route this through another provider for the redirect and serve a valid cert. Sorry it didn't happen immediately - it was a "minor" domain and just wasn't the highest priority at the time.
This is no longer relevant, since stackpromos.com domain does not exist anymore, and redirecting to https://contests.stackoverflow.com instead, which being part of stackoverflow.com domain, gets its https cert.