Another thought. One of the things that I did was to set the burn speed at 2x, even though my DVD-Rs are rated at 4x. I found that recording at 4x (on my laptop) or at "Max" speed didn't always result in readable discs on some DVD players. Since the file size is small, it doesn't take very long anyway.
It's strange that your RP91 won't read your DVD-R. I have a number of TV movies that I copied (PVR on my PC) and burned to DVD-Rs. I've had no problem playing them on my RP91. I'm trying to remember when I got mine, but I think it was around 6-9 months after they became available.
BTW, I've replaced my RP91 with a Sony DVP-NS75H upscaling player. It does a very good job over the component connections (480p - which I needed) but doesn't have the adjustments that the RP91 has. In hind-sight, I should have waited another month, or so, to get the Oppo 970H, since it's a better player (and doesn't have some of the limitations that the Sony has). Oh, well, I'll wait for the HD players to sort themselves out before my next DVD player upgrade.