Same issue with RC1 -- I somehow corrected it with Command+R restart. Same issue occurred with RC2 -- the previous method failed. I am stuck and disappointed.

I figured out an easy way to return to RC2: option+start key, then select the main system disk (not the installation). In my case, my machine booted into RC1. I am tired of trying this and will deregister. Hopefully I can go from RC1 to the release tomorrow!