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Here is the log:

INFO: Launching wrapped minecraft {net.minecraft.client.main.Main}
[18:06:20] [Client thread/INFO]: Setting user: XxPocket_LintxX
[18:06:20] [Client thread/INFO]: (Session ID is token:86718824e79b4946be2edd75be921933:74b3c75538dd43d8b25de09af6c8b886)
Apr 29, 2015 6:06:20 PM net.minecraft.launchwrapper.LogWrapper log
SEVERE: Unable to launch
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Caused by: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.Gson.fromJson(Gson.java:815)
at com.google.gson.Gson.fromJson(Gson.java:768)
at lu.b(SourceFile:151)
at lu.<init>(SourceFile:43)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:155)
at bpp.<init>(IntegratedServer.java:52)
at avf.<init>(SourceFile:293)
at net.minecraft.client.main.Main.main(SourceFile:124)
... 6 more
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:338)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:79)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:60)
at com.google.gson.Gson.fromJson(Gson.java:803)
... 13 more

Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

Can anyone tell me how to fix this?

Remove the file %appdata%/.minecraft/usercache.json