Hi Addison,

I replied earlier but must have managed to delete my reply somehow. I had this happen recently when setting up the Web Client in the DMZ, and the fix was allowing sufficient permissions to the certificate(s) in question for the relevant IIS app pool. In my case, it was the WebClientAppPool. I honestly don't remember if it was the SSL certificate in my case or the certificate used to create the trust relationship between my LFDS machine and my web server. But this was the fix with the exact behavior you're experiencing.

If you open mmc.exe, add the Certificates add in, find your certificate(s) and right-click, and then click 'Manage Private Keys,' you'll be able to add permissions for your FormsAppPool. 

I'm interested in what the solution is here if this doesn't help. Keep me updated!