1.6.1 will not load my world!!!

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!!