My DVD (a Philips DVP5980) has decided that I have to enter a password before I play a particular DVD - the second disc in part one of the Lord of the Rings. The trouble is, I never set any password

My assumption was that somehow I must have accidentally set some parental lock thing - but then I found that it's only on this DVD that it asks for a password. Other DVDs are fine. At least all the ones I've tried it with are.

I've tried unplugging the DVD to reboot the machine, but that doesn't make any difference.

Any suggestions as to what the problem is, and how I can sort it out?