When you play on a server it's fine, but when you leave and try to reconnect it kicks you with internal server error.

Server Log:
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27 achievements
2013-05-04 08:27:46 [INFO] Starting minecraft server version 13w18b
2013-05-04 08:27:46 [INFO] Loading properties
2013-05-04 08:27:46 [INFO] Default game type: ADVENTURE
2013-05-04 08:27:46 [INFO] Generating keypair
2013-05-04 08:27:46 [INFO] Starting Minecraft server on *:25565
2013-05-04 08:27:46 [INFO] Preparing level "Server Survival"
2013-05-04 08:27:46 [INFO] Preparing start region for level 0
2013-05-04 08:27:46 [INFO] Done (0.586s)! For help, type "help" or "?"
2013-05-04 08:27:47 [SEVERE] Reached end of stream
2013-05-04 08:27:49 [INFO] scottipod2000[/98.228.100.85:48475] logged in with entity id 228 at (273.53683186896694, 54.0, -264.6487365532736)
2013-05-04 08:27:49 [INFO] Disconnecting scottipod2000 [/98.228.100.85:48475]: Internal server error
2013-05-04 08:27:49 [WARNING] Failed to handle packet for scottipod2000 [/98.228.100.85:48475]: u: Sending packet
u: Sending packet
at jj.b(SourceFile:443)
at gw.a(SourceFile:121)
at gw.a(SourceFile:76)
at jh.d(SourceFile:161)
at jh.c(SourceFile:53)
at ih.a(SourceFile:44)
at ig.b(SourceFile:29)
at net.minecraft.server.MinecraftServer.s(SourceFile:490)
at ib.s(SourceFile:262)
at net.minecraft.server.MinecraftServer.r(SourceFile:407)
at net.minecraft.server.MinecraftServer.run(SourceFile:341)
at gr.run(SourceFile:584)
Caused by: java.lang.NullPointerException
at fh.a(SourceFile:66)
at ck.a(SourceFile:159)
at jj.b(SourceFile:426)
... 11 more
2013-05-04 08:27:49 [SEVERE] Reached end of stream
2013-05-04 08:27:50 [SEVERE] Reached end of stream
java.lang.IllegalArgumentException: Empty string not allowed
at cg.<init>(SourceFile:15)
at bu.a(SourceFile:77)
at asa.e(SourceFile:186)
at asa.b(SourceFile:112)
at alh.a(SourceFile:88)
at alh.a(SourceFile:76)
at jb.a(SourceFile:646)
at jb.a(SourceFile:631)
at net.minecraft.server.MinecraftServer.a(SourceFile:265)
at net.minecraft.server.MinecraftServer.r(SourceFile:412)
at net.minecraft.server.MinecraftServer.run(SourceFile:341)
at gr.run(SourceFile:584)
2013-05-04 08:33:01 [INFO] scottipod2000[/98.228.100.85:45276] logged in with entity id 231 at (273.53683186896694, 54.0, -264.6487365532736)
2013-05-04 08:33:01 [INFO] Disconnecting scottipod2000 [/98.228.100.85:45276]: Internal server error