When player hops into Minecart, he is immediated crashed and the Launcher has Play greyed out as if he is still in game. The player cannot reenter game even if Launcher and server are restarted. Previous backups of the player can reenter game, but will crash again if he hops into a minecart.

Here are the errors:
From server:
[INFO] brewmaj[/172.16.1.114:53025] logged in with entity id 78171 at (-668.5099999904633, 10.699999988079071, 527.5)
[SEVERE] Reached end of stream
[INFO] brewmaj lost connection: disconnect.endOfStream

From Launcher 0.2:
Client> 2013-04-20 10:40:13 [CLIENT] [INFO] Connecting to 172.16.1.114, 25565
Client> java.lang.NullPointerException
Client> at ek.n(SourceFile:174)
Client> at ck.a(SourceFile:237)
Client> at ck.a(SourceFile:225)
Client> at ck.h(SourceFile:175)
Client> at ck.d(SourceFile:16)
Client> at cm.run(SourceFile:115)
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> at kq.a(SourceFile:237)
Client> at bhq.run(SourceFile:114)
Client> at java.lang.Thread.run(Unknown Source)