finally, I found the solution:
in fedora 26 / 28, "java-1.8.0-openjdk-headless-1.8.0.181.b15-0.fc28.x86_64" is installed by default.
the package "java-1.8.0-openjdk-1.8.0.181.b15-0.fc28.x86_64" needs to be installed in addition, then JDownloader runs.
so, it was a Java installation problem, and not one of wayland.
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