Lots of web service vendors these days are tightening their requirement security protocols, SSL was removed back on the hearthbleed days, and not TLS version 1.0 and 1.1 are getting outed too. So suddenly NAV code that has been working for years stop working unless you change your client to select TLS1.2. Thankfully, there is a simple trick to choose the security protocol used by DotNet. This is done with the ServicePointManager.SecurityProtocol property:
Set it to SecurityProtocol.Tls12
Now you’re talking Tls12 with DotNet from inside NAV.