JDownloader was working properly and than you just get core dumps all over again? Best try, upgrade your Java Runtime Environment.
sudo archlinux-java status sudo pacman -S jre9-openjdk sudo archlinux-java set jre9-openjdk #if not needed anymore sudo pacman -R jre8-openjdk
You need to update the $PATH variable. Either you restart your system or you open a new terminal to start JDownloader from their. All should work fine now.