@SteveManes I had a similar problem. I'm also sharing these cross posts.
I'm running macOS 10.15.7.
I saw the "UD Loading" screen and then... nothing, the application quit.
I think it was due to an expired certificate or some type of check within the ISY jar launcher, but all of the standard options I tried didn't have any effect (including some more aggressive ones, like removing the Java apps etc.).
Finally, here is what worked for me based on some advice in other posts:
1. Run the following commands to clear the old version of Java (use caution, since these are with sudo)
sudo rm -fr /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin
sudo rm -fr /Library/PreferencePanes/JavaControlPanel.prefPane
sudo rm -fr ~/Library/Application\ Support/Oracle/Java
2. Download and install Java 251 from here https://www.oracle.com/java/technologies/javase/javase8u211-later-archive-downloads.html
(Not sure if it matters, but I also ran the uninstaller first)
3. Now, use the start.jnlp command. ISY will load properly! (Ignore the out of date Java warning for now)
4. Upgrade Java back to 261.
After these steps, everything works for me.