In at least Ubuntu and Mint there is nothing intuitive about configuring Java.
Execute it to get to it:
You'll be able to configure different options and variables.
You can also check under Security as some of those settings could prevent applets from starting or prompting to........
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.........