When I start up Minecraft 17w50a I see a black window open up, the title is Minecraft 17w50a. after about 0.5 secs the window closes and im back in the launcher with an error message:

The game crashed whilst initializing game
Error: com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Invalid escape sequence at line 4 column 38 path $.pack.description

Glad if someone could help me