Whenever I try to convert an old world, the game crashes upon trying to do it. Even on a clean version it happens. It fails because of a "Corrupt NBT Tag." The file goes as such:
---- Minecraft Crash Report ----
// You're mean.

Time: 15/03/13 11:39 AM
Description: Reading NBT data

java.lang.ClassCastException: bz cannot be cast to cb
at bs.f(SourceFile:134)
at aby.a(SourceFile:30)
at ajk.a(SourceFile:215)
at ajk.a(SourceFile:186)
at ajk.a(SourceFile:145)
at net.minecraft.server.MinecraftServer.b(SourceFile:112)
at bja.a(SourceFile:59)
at bja.c(SourceFile:104)
at net.minecraft.server.MinecraftServer.run(SourceFile:306)
at gp.run(SourceFile:573)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Corrupt NBT tag –
Details:
Tag type found: INT
Tag type expected: LONG
Tag name: LastUpdate
Tag parent: Level
Stacktrace:
at bs.f(SourceFile:134)
at aby.a(SourceFile:30)
at ajk.a(SourceFile:215)
at ajk.a(SourceFile:186)
at ajk.a(SourceFile:145)
at net.minecraft.server.MinecraftServer.b(SourceFile:112)
at bja.a(SourceFile:59)
at bja.c(SourceFile:104)
at net.minecraft.server.MinecraftServer.run(SourceFile:306)
at gp.run(SourceFile:573)

– System Details –
Details:
Minecraft Version: 1.5
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_17, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 461071952 bytes (439 MB) / 514523136 bytes (490 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Profiler Position: N/A (disabled)
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.