Remote certificate was verified as invalid by the user
Are you providing a ServerCertificateValidationCallback anywhere else in your code?
We had a logical flaw in a callback that we implemented which was whitelisting specified domains with self-signed certificates. Namely, the callback was always executing - even for valid certificates - but only applying the whitelisting logic. Because legitimate certificates weren't in this list, the callback was indicating failure.
This was resolved by returning early based on the error variable:
if (error == SslPolicyErrors.None) return true;