JDownloader may restart itself several times. For example when installing an update. During startup the headless version of JDownloader will auto check for updates and install them. That's why it sometimes work and other times don't work.
So the PID from first start may no longer be valid.
try adding to [Service]
PIDFile=/home/pi/bin/jdownloader/JDownloader.pid
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JD-Dev & Server-Admin
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