Welcome to the forum.
This unit has two pin numbers associated with it: The owner identity PIN, and a separate parental control PIN.
You probably put a pin number in on original setup for owner identity...but it seems likely that you have never input a parental control pin, so it
should still be at the default 0000, albeit you say you have tried that.
The parental control pin is only required for titles that have a 'G' next to them in the list of recorded titles. Is it possible that you have encountered such a title for the first time?
The requirement for a parental pin can be turned off for all titles (see page 70 of the manual)... but the pin number is required to change this setting (Catch 22).
You could try changing the setting using the expected 0000 pin.
If that fails, try a power reset ( mains disconnection for 10 minutes ) and try again.
If that still fails, reset the parental Pin number by:
1 - While the unit is on, press and hold [OK], the yellow button and the blue button at the
same time for more than 5 seconds.
• “00 RET” is displayed on the unit’s display.
2 - Repeatedly press (right) until “03 VL” is displayed on the unit’s display.
3 - Press [OK].
• “INIT” is displayed on the unit’s display.
• The PIN number for parental control returns to the factory preset (“0000”).
Good luck.