I actually had this happen recently on one of our tracking servers... I thought it was because it was old and set up before my time... Client Care had no record of our tracking server set up for SSL.
I am now wondering if Adobe accidentally dumped old servers accidentally (since the CNAME process is quite different now). And because they were so old (the one of ours that broke was done like 10 years ago), they didn't have it properly documented.
Anyway, the solution for us was to contact support and get the server re-setup with a proper SSL CNAME, then I had our DevOPs make the DNS change.
Good Luck