it may simply be that your panasonic recorder cannot handle +r DL.
Panasonic dvd recorders have a reputation for being fussy. I owned one ( different model number to yours) which would use +r or - r itself, but would not play any -r from my PC unless that disc was 100% full. And it would not record to a +RW that I’d erased on the PC, it had to be fed a +RW with old content and allowed to do the erase itself before it would reuse.
So to sum up I think the Panasonic has problems with the unused parts of some formats & decides that It cant read the disc. IF your DL disc reads OK on another player and OK on your PC then it’s fair to assume that the disc has been written to DVD specs and that it’s the Panasonic that’s at fault. Maybe look around for a PAnasonic forum & ask there ?