DVD's not playing

My DVD player does not recognise regular DVD's.
I get the meassge "Unknown disc. Please, check disc again" and it spits the disc out.
This happens with brand new DVD's and old ones. I've tried cleaning the player which has not made any difference.
I reinitialized the whole sytem back to factory defaults and one dvd played, but I tried another and nothing again, went back to the first one and again nothing.
This doesn't happen with DVD's I've recorded myself which play fine, so it's not the player itself but some sort of random setting that I can't figure out.
Any ideas would be greatly appreciated.
Model no. RH2T80