PayPal Error: The certificate chain did not validate, common name did not match URL.

I have a .Net web app that processes payments. The payments are processed by our Association Management Software, named Imis. The assemblies for Imis are in the local bin along with the paypal assemblies (payflow_dotnet.dll, etc.)

The application stopped processing transactions a few days after July 1, 2018. After I started digging around I found that one of our other web apps that was on a different server WAS processing payments just fine. So I copied the broken app to the other server and it worked fine. I copied the complete root directory and just set up a new site in IIS.

When the payment fails, the response from PayPal is "Error processing payment: The certificate chain did not validate, common name did not match URL. Input Server Uri". One thing that somewhat sticks out to me is that in some of the other articles that display this message, there is a Uri at the end of the error message. Mine is blank. I wonder if that is the issue, that the request is not sending the Uri as expected. But I don't know what would affect that.

The server where things work is Win2008r2. The server where things DO NOT work is Win2012r2 Datacenter. Both have .NET 4.5.2 or better installed. We have installed all the windows updates we can. We found a tool that showed what protocols were in play on the machines and both were the defaults. We checked the registry key where you can enable and disable certain ones and they both had the exact same entries.

I feel the sites are the same, I just don't know what else to look for that would be different between the 2 servers. It doesn't seem to be code related because I have the same code base running on one server just fine.

Any thoughts on how to proceed with troubleshooting past what we have done? I'm at a loss.