Kicked from server when in certain area: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1

Summary:
I had a server on 1.8 with a few friends. Today I upgraded it to the first 1.9 snapshot and kept the world.

When entering the server I see the world for ca. 0.1 sec then the following error message:
"Connection Lost

Internal Exception: io.netty.handler.codec.DecoderException: com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 2" (and then the button "Back to server list")

I then tried this a few times and then started a new server which worked but as soon as I placed down a sign I got kicked again. I was not able to edit the sign.

What I expected: No error message.

What actually happened: see error message above.

Steps to reproduce:
Start Minecraft Launcher.
Download Minecraft Server from mojang.com.
Start Minecraft server on same Computer.
Log into minecraft.
Click "Multiplayer".
Click "Direct Connect".
Enter 127.0.0.1 and Click "Join Server".
Place down sign.

Server output:
http://pastebin.com/ey3N1fcz

Server config:
http://pastebin.com/KSZAh1Zk