Moving to latest version(s), but now will not load previously created world. New launcher launches 1.6.1, says Loading..Building.. then freezes, then (sometimes) craps out to Title screen.

What I expected to happen was...:
Should load world as before.

What actually happened was...:
As described, it freezes after "Building...". The Launcher also freezes – can't scroll in the development box, for example. Can see it listed "severe" errors, though – "Wrong Location..." (I did a screen grab.)

Steps to Reproduce:

• launcher 1.0.8 runs
• click "Play"
• click "Single player"
• select world – "Liam's Creative World"
• click "Play selected world"
• displays "Loading... Building..." then freezes, then eventually craps out to Title

This is for my son, 8 years old, now passionate about Minecraft. He's built lots of stuff and doesn't want to lose it all. World loaded fine prior to this version. Please help!!