Did you change a .css file when you were in PR 1.0? It is maybe not valid anymore with the PR 1.1.

I had the same problem: I made some modifications in the meegotouchhome.css file (colors, font size) when my N9 was in PR 1.0. With the new firmware, this file was slighlty different, and after the first reboot, the phone could not start anymore.

So I had to reflash (Navifirm...), and to made my mods once again on the new meegotouchhome.css file.