Adobe reader DC saying "invalid SSL certificate"

Hi,

I built a form with a submit button that submits to my internal server over HTTPS. Everytime I click submit, it says "the server has an invalid SSL certificate" but I don't understand why because the SSL cert is valid, I just checked it with www.ssllabs.com and my score is an A (validating that everything is correct). Why would adobe be saying this? Is there a log or more detail I can get as to why it think the cert is invalid?