org.jdownloader.update.launcher.JDLauncherFailedException: java.lang.NoClassDefFoundError: org/jdownloader/startup/Main
	at org.jdownloader.update.launcher.SecondLevelLauncher.launchJDownloader(SecondLevelLauncher.java:743)
	at org.jdownloader.update.launcher.SecondLevelLauncher.init(SecondLevelLauncher.java:496)
	at org.jdownloader.update.launcher.SecondLevelLauncher.runMain(SecondLevelLauncher.java:230)
	at org.jdownloader.update.launcher.JDLauncherViaClassLoader.main(JDLauncherViaClassLoader.java:10)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.jdownloader.update.launcher.JDLauncher.main(JDLauncher.java:79)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at com.exe4j.runtime.LauncherEngine.launch(Unknown Source)
	at com.exe4j.runtime.WinLauncher.main(Unknown Source)
	at com.install4j.runtime.launcher.WinLauncher.main(Unknown Source)
Caused by: java.lang.NoClassDefFoundError: org/jdownloader/startup/Main
	at org.jdownloader.update.launcher.SecondLevelLauncher.launchJDownloader(SecondLevelLauncher.java:683)
	... 15 more
Caused by: java.lang.ClassNotFoundException: org.jdownloader.startup.Main
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	... 16 more

After

mega.nz/#!mctCnIhC!fNqx1UOf06cVB7h9Cis1Kk5J_tAGMTFsSQcuQ2FPfYw
and replace "JDownloader.jar" with the downloaded one
Delete "Core.jar" and the "tmp" and "update folders" and try again

[Jiaz]

Did you really work in correct folder?
should be within %appdata% on windows by default, not program files!
Also make sure that you've deleted Core.jar AND the folders tmp and update.
And replace JDownloader.jar with the downloaded one.

Only uninstall and clean Installation fixed.

[Jiaz]

Thanks for the feedback! The steps in post 2 would have worked as well