I am not familiar with that KVM Viewer, but here are some questions and debugging steps that might help figure it out. First, is it failing on all systems or just some of them? It looks like you're using TLS, am I right? Also, are you using digest or kerberos credentials?
For debu, try it again on a failing system. Then, try AMT's webUI (https://<FQDN>:16993). Be sure you can connect, that there's no cert error, and that you can log in.
If that works, something is most likley going wrong with your viewer. For debug, try RealVNC Viewer plus. It has a 90 day trial license.
If the web UI fails to connect, try pinging the AMT system by IP and FQDN to be sure it's reachable and that the DNS <-> IP relationship is right. Note: Windows firewall will block pings, so turn it off temporarily on the AMT system.
If the cert or login fail on the webUI, then that's most likley the issue with the viewer as well.