I just downloaded all the downloads of the NB5.5 distribution, so that would be NB55 and all additional packs.
I then uninstalled all previous versions of NB, which were NB55 dev, RC1 and RC2 and the M3 release of NB6. Then all userdirs were removed from my system as well. So you can see I've got a clean NB environment. And on with the installation...
Everything worked fine and everything installed fine. Then it was time to start NB55. Finally it's been released. But surprise surprise, the