I had the same problem of not being able to get into admin also. I could log in to the
VB admin and the VBP navbar login would show that I was logged in but, PP would show 1 unregistered user. I check my settings for cookie prefix and cookie domain through phpMyAdmin and they were correct. I just happened to notice in the table "settings" the user registration was still showing vB2. I changed it to vB3 and all is ok.. So this maybe another slight problem that some are coming across.
Scott