Solved: Jdownloader2 not showing any GUI

I have trouble to run Jdownloader2 on one system (laptop and older desktop work perfect). I did a fresh installation from AUR and when I select Jdownloader from the Applications/Internet menu, I see that java starts - but no Jdownloader GUI is showing up.
If I open a terminal in .jd folder and run java -jar JDownloader.jar I see that one instance of Jdownloader is running.
What can I do to debug this further?
(It may be a misconfiguration in java, I tried to install Arduino and also no reaction after selecting the app)
Best Regards, Andreas

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