This error is commonly due to Java security or TLS settings but there is a second issue with forwarded ports that also causes it.
1. Java Security/TLS Settings issue:
This article has the solution to change them all in Linux automatically
2. Port Forwarding Issue if your Avocent DSR is behind NAT/private IP........
This command in Debian/Ubuntu/Mint will get everything need installed for most Java based KVM viewers:
sudo apt install icedtea-netx
The following additional packages will be installed:
ca-certificates-java icedtea-netx-common openjdk-8-jre openjdk-8-jre-headless
It seems every other updated version of Java or Icedtea breaks things and I will save the frustration of Java for another post.........
Avocent Unable to load resource avctVideo.jar
Avocent requires port 80 and 443 to be forwarded or you will get a message like above if it's on a NAT network and you are accessing it from the outside (internet).........