Thank you Zero and OvermindDL1 for your concern.
If I delete the META-INF directory, java will not be able to know which class the main-method is, and thus will refuse to start anything.
Please tell which class-file I should give as having the main method (I have used LauncherFrame.class which give me the "usual" login).
Also (as stated by 3), I have unpacked all files to a directory and started the LauncherFrame directly (in this case, the META-INF directory is ignored by java), but with the same error as originally is reported.
Sorry, but I think I am still stuck.