If you are looking for the decompiler itself, visit https://github.com/jindrapetrik/jpexs-decompiler
List of issues
: The issue tracker is now writable again and logged users can download files too. But note that our support is very LIMITED.
#1537 Alternate JREs not working
> What steps will reproduce the problem? Uninstall Oracle's JRE, Install OpenJDK, start Flash decompiler > What is the expected output? What do you see instead? I expect flash decompiler to start. Instead I get an error message stating it requires a Java 8 JRE and opens a new tab in the browser to Oracle's JRE > What version of the product are you using? Is it "nightly build"? Which operating system do you have? 11.2 > Please provide any additional information below. If the problem is related to a SWF file, attach it here, otherwise we can't help you.
You can use ffdec.bat or ffdec.jar to launch FFDec using your default JRE. I assume you are using Windows and starting it using ffdec.exe - it's just a launcher which might not work in some cases.
Tried that, didn't work. Using AdoptOpenJDK with HotSpot 126.96.36.199
Tried AdoptOpenJDK with Eclipse OpenJ9, that works.