I have posted to aid others in diagnosing problem with SSL/TLS connections to superserver port from .NET client executable.

The cache instance this appeared on is quite old - 2011 - so I do not know if Intersystems have added a better error message in a later version

The actual fault was due to the certificate in the %SuperServer SSL/TLS configuration having expired.

The unhelpful message that appeared in the .NET client included the following partial stack trace.