I had the same problem today with my DCX-3510M. I recorded several episodes of Dr. Who yesterday on the Space channel (Ch 39) yesterday, but five episodes that looked fine yesterday were "Locked by Rating" when I tried to watch them today. I was asked to enter and confirm a PIN when I selected "Watch from Beginning", but I decided to exit and unplug the PVR to solve the problem.
I checked my parental control settings and all were set to their defaults (Max movie rating: A; Max TV rating: 18+; Lock non-rated movies: No).
I've actually had more problems with my DCX-3510M since the firmware update to v. 11.x in November 2013. Many are identical to those I previously reported for the old firmware (e.g., double recording of episodes, episodes not recording to completion, etc.) but a few like the "Locked by Rating" problem are new to v. 11.x. I don't know if this has anything to do with the fact that I have a standard def TV but my 3-month introductory offer with Shaw is now finished and I'm incredibly disappointed with with the service so far.