An .NET app threw the above error when it tried connect to a FTP server over TLS/SSL.
In this case, the error was caused by the SSL certificate. The SSL certificate was a Multiple Domain (UCC) SSL Certificate. It was issued for ftp2.mydomain.com at first. When ftp1 server was setup, I had to add a SAN (Subject Alternative Names).
I made the mistake of entering “ftp1” instead of a fully qualified domain name “ftp1.mydomain.com”. That triggered the error in the Title.
The solution is simply to drop the “ftp1” and add “ftp1.mydomain.com” SAN, reissue the SSL, and install the SSL onto ftp1 server.
Note: ftp2 server also needs to reinstall the newly issued SSL certificate since the old SSL certificate will expire in 24 hours.