Update: This is likely an ssl issue. If your host uses cpanel autossl it does not contain the correct key to work with Pinterest. I am trying to figure out what and why and how to fix this. But a quick, easy and free fix to this pin problem is cloudflare. Setup your site using their free option. Make sure your dns matches what is in cpanel and then setup your ssl certificate through cloudflare. Give it about 30 minutes to an hour to propagate and you will be able to pin! Just did this for a client of mine and it worked.
You can check your ssl at https://www.ssllabs.com/ssltest/. I did this before and the autossl recieved a B. I ran it again after the change to cloudflare and it still picked up the original autossl. It again gave it a B, but it also picked up the new ssl cert and it received an A! All good on her site now.
