Sorry, I am an old school java end-user, and was always told that JDK was for developers and JRE (runtime) for end user. Didn't noticed that Oracle ended JRE at version 8 ann I thought that there last JRE8u392 was teh corresponding runtime to their 21 JDK ... Thanks for pointing me. Maybe a link on the Ubooquity website would help some other dumb JRE users like me.
By the way, I discovered that Eclipse still offers an up-to-date (based on OpenJDK 21) JRE.

Ok it works with Java 21 SE. Nice update. Still wishing for RAR 5. Keep the good work. Thanks

I understand that, however that version is the only one oracle is pointing to when looking for the JRE. I Have found nbo way to download somthing like a Java 17 JRE. See what happens when one is looking for the customers java download : https://www.java.com/en/ (clik on the Download link, it will redirect you to JRE 8.xxx.
I must be missing something. Has the JRE been replaced by something else ? 

Was using jre-8u381-windows-x64 when first reporting the crash. I upgraded today to jre-8u391-windows-x64 and still crashes. Tested on another computer and same problem.
I must admit I don't know where to find the crash log ...
Only thing I can tell is that the crash message comes from "Java Virtual Machine Launcher", that the previous Ubooquity works great on theses computer, only the 3 beta crashes, and that on the other computer there is another message precising that the problems seems to come from "JNI" initialization system.

Hi, very happy to see this rising again from ashes ... !

Hope it will ultimatly accept rar5 .cbr.

In the meantime, it crashes (on my x64 Win 11 with the latest java RE) at start with "A java Exception has occurred" error.