5.5.0 to 5.5.0b Upgrade fails

Hi.

I'm trying to upgrade from 5.5.0 to 5.5.0b.

As normally (and what was done when upgrading from 5.1.0a to 5.5.0) and ran Simple Install.

It fails with the following error:

Any Clues?

And I'm certain that I have mounted the correct ISO.