Launcher won't open MC 1.8 for no reason

When i push at start on the new Launcher it just Closes ist window and creates the game Output window. This puts out this:

13:43:47 launcher main info Preparing to launch minecraft client for 1.8
13:43:48 launcher main info Checking installations.
13:43:48 launcher main info Minecraft client 1.8 is ready to start.
13:43:48 launcher main info Starting!
13:43:49 launcher main info Using default game log configuration client-1.7.xml (outputs XML)

After this the launcher opens and tells me that theres a problem occured.
Can you pls help me :*