NullPointerException on connection

After upgrading to 1.4.6 there was about 5 minutes of successful gameplay, and then all players were disconnected after the server reported NullPointerException stack traces.

Subsequent attempts to connect are all unsuccessful. The server console reports:

2012-12-20 16:48:04 [INFO] Gangoggs[/10.0.1.7:63961] logged in with entity id 142 at (-44.30000001192093, 87.0, 335.66222514255367)
java.lang.NullPointerException
at ef.a(SourceFile:310)
at mj.a(SourceFile:237)
at mj.a(SourceFile:157)
at et.a(SourceFile:36)
at ef.a(SourceFile:214)
at cg.h(SourceFile:178)
at cg.d(SourceFile:15)
at ci.run(SourceFile:114)
2012-12-20 16:48:04 [INFO] Gangoggs lost connection: disconnect.genericReason

This user was in creative mode. Another user in survival mode elicited the same response. The stack trace is identical on both operating systems (Linux and OSX).