I have recently run into this problem too even though I have first run the vanilla launcher to update the game libraries. The problem still exists.
In fact, I have found that it's showing a bad jar file indicator for me within Magic Launcher even if I try to re-specify the proper jar location and file. I have even tried removing Magic Launcher's configuration file from the "magic" folder within the roaming appdata location. Nothing has worked to fix it so far.
Still having this problem with Minecraft Version 1.8.4.
It only happens with "Environments" (for the "Environment" Setting) above Minecraft Version 1.8.