Hmm, weird. Maybe the Xorg logfiles could be helpful. Does sddm-greeter write a logfile? The backtrace at least mentions QMessageLogger::fatal(char const*, ...) const so there should be some output somewhere.
Does any other display manager work with 0.6.0? (kdm seems to work for me with the changes from 0.6.0). Can you log into the console/ssh into the machine even if sddm crashes? The information collected by 'reportbug nvidia-driver' after the crash should help debugging (don't submit that as a new bug report, but save it and send the info here) Andreas
