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Does this happen with blank or burned DVD+Rs? Does an error show up on screen/on the display?
I had a Philips DVD recorder show similar symptoms after a year and a half’s use. Updating the firmware had no effect, as in your case…
Whenever I put a burned DVD-R/DVD+R into it, it would display “Empty Disc”, as though the disc was blank.
Only discs it would play successfully in the end were DVD RWs (with data on - empty DVD RWs always showed “Disc Error” ).
I did comtemplate opening it up, and either manually cleaning, or even swapping out the drive completely, but in the end just bought a new unit as I couldn’t be bothered messing about with it ;).
Have you tried cleaning the lens? First thing I would try is a lens cleaning disc.