[MC-102741] crashes on opening

minecraft should launch
it crashes and reverts back to the 'game output' tab of the launcher screen
with

09:19:24 FATAL]: Fatal error launching game. Report this to http://bugs.mojang.com please!
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:176) ~[launcher.jar:1.6.61]