Hi!
First of all, have you tried mutiple discs in NTSC-format?
I have had a NTSC-disc that wouldn't play in this particular model, I don't know the exact reason, but the disc itself felt a little bit thinner than most other discs.
I'm no expert, but I would try to give you an advice. Do you feel confident to open the player and look for visible faults in the electrical parts? I'm not sure if there is any electrical schematics available on this forum for this particularly model, but most likely there is a fault related to the ability to play foreign discs. It could be the chip itself that does the conversion of NTSC.
The best solution would be to replace the player with a higher end model, unless you want to spend countless of hours of troubleshooting that may not end up with a fixable result in the end.