Client unable to log into snapshot 15w31b server

Please try resetting the server world (usually the `world` folder) and the `usercache.json` file.

Are you asking that I wipe my world and start fresh? While I can do that, the idea would be to be able to upgrade an existing world so the players to not have to start from scratch. Please be more specific on what you would like me to do. Since I'm working with a dev environment I can do anything you need for this. I would rather not have to lose all world progress with 1.9 proper hits the streets.

Please confirm that creating a new world does not experience the issue, and if possible upload the world for testing. If the world is too big please tell me how large it is so I can find a place for you to put it.

Was Client / Server running under 15w31a or updated from 1.8.x to 15w31b ? May be duplicate of MC-82968

The client/server never ran 15w31a but did upgrade from 1.8.8 to 15w31b. I have multiple profiles to switch between 1.8.1, "Current Release" and Snapshot for the client. The 1.8.1 and Current Release (1.8.8) operate as expected, Snapshot 15w31b does not. There production server still run 1.8.1.

Moving world and usercache.json did allow for a playable game though:

terminal log

$ mv world world-orig
$ mv usercache.json usercache.json-orig
$ java -Xmx1024M -Xms1024M -jar minecraft_server.jar 
[02:41:27] [Server thread/INFO]: Starting minecraft server version 15w31b
[02:41:27] [Server thread/INFO]: Loading properties
[02:41:27] [Server thread/INFO]: Default game type: SURVIVAL
[02:41:27] [Server thread/INFO]: Generating keypair
[02:41:28] [Server thread/INFO]: Starting Minecraft server on *:25565
[02:41:28] [Server thread/INFO]: Using default channel type
[02:41:28] [Server thread/INFO]: Preparing level "world"
[02:41:28] [Server thread/INFO]: Preparing start region for level 0
[02:41:29] [Server thread/INFO]: Preparing spawn area: 4%
[02:41:30] [Server thread/INFO]: Preparing spawn area: 7%
[02:41:31] [Server thread/INFO]: Preparing spawn area: 11%
[02:41:32] [Server thread/INFO]: Preparing spawn area: 17%
[02:41:33] [Server thread/INFO]: Preparing spawn area: 26%
[02:41:34] [Server thread/INFO]: Preparing spawn area: 32%
[02:41:35] [Server thread/INFO]: Preparing spawn area: 40%
[02:41:36] [Server thread/INFO]: Preparing spawn area: 49%
[02:41:37] [Server thread/INFO]: Preparing spawn area: 57%
[02:41:38] [Server thread/INFO]: Preparing spawn area: 65%
[02:41:39] [Server thread/INFO]: Preparing spawn area: 73%
[02:41:41] [Server thread/INFO]: Preparing spawn area: 80%
[02:41:42] [Server thread/INFO]: Preparing spawn area: 87%
[02:41:43] [Server thread/INFO]: Preparing spawn area: 94%
[02:41:43] [Server thread/INFO]: Done (15.470s)! For help, type "help" or "?"
[02:41:43] [Server thread/INFO]: Starting GS4 status listener
[02:41:43] [Query Listener #1/INFO]: Query running on 0.0.0.0:25565
[02:42:22] [User Authenticator #1/INFO]: UUID of player turtle2472 is xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
[02:42:22] [Server thread/INFO]: turtle2472[/127.0.0.1:53482] logged in with entity id 1080 at (-222.5, 64.0, 7.5)
[02:42:22] [Server thread/INFO]: turtle2472 joined the game
[02:42:27] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3460ms behind, skipping 69 tick(s)
[02:42:57] [Server thread/INFO]: [turtle2472: Set own game mode to Creative Mode]
[02:43:31] [Server thread/INFO]: turtle2472 has just earned the achievement [Taking Inventory]
[02:43:48] [Server thread/INFO]: turtle2472 lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[02:43:48] [Server thread/INFO]: turtle2472 left the game

I was able to place a sign and edit it unlike MC-82968. This issue seems more related to the upgrade.

This is a link to my world folder as of 1.8.8: https://dl.dropboxusercontent.com/u/2306664/Archive/world.zip 388.9MB
This is a link to my world folder as of 15w31b: https://dl.dropboxusercontent.com/u/2306664/Archive/world_15w31b.zip 388.9MB

Please let me know if you would like the entire MC operating dir.

I have exactly the same problem.
I tryed updating my 1.8.8 server to snapshot 15w31b (and c today) and i am getting
the same json parsing error when i try to login.
I also didn't use 15w31a before this

The error message "... to accept malformed JSON at line 1 column 2" seems to have the same root cause like MC-82968