I was getting this error when I had deployed my code to two load-balanced servers, but only on one of the servers. It would work on the other server. I think the important thing here is to import your certificate to My/LocalMachine so that you have the ability to manage the private keys. I deleted my certificate in the other locations before doing this. The weird thing was that the certificate was set up on the servers identically, and I was able to confirm that the private keys were also in the same relative location using the FindPrivateKey tool. On the server that was causing the issue, I simply:

1. Opened the MMC on the server in question.
2. Added certificate snap-in for local machine.
3. Right clicked on my certificate and selected "Manage Private Keys".
4. Clicked "Add"
5. Clicked the "Locations" button and selected the machine name at the top.
6. In the object name box I typed in "Users" and hit "Check Names"
7. Clicked "OK"

It's weird that both servers didn't require this, but I thought it was worth mentioning that sometimes server configuration has inconsistencies that need to be accounted for.