Can't play store bought DVDs

I recently had my Superdrive replaced by an official AppleCare center, due to some previous problems.
Now, I just can't play most (not all, just most) of my DVDs using DVD Player.
The playing process starts fine but after the initial Warning/Disclaimer and producer's logos, when it comes to getting to the dvd menu, I get this error messsage:
"the disc copy protection could not be verified -70025"
The same discs work fine on my old PowerBook G4, and can still be played on my MacBookPro using VLC, but that's not the best option, since DVD Player.app is a much better program.

I have OS X 10.4, and here's the system profiler data for my computer and new optical drive:

Model Name: MacBook Pro 15"
Model Identifier: MacBookPro1,1
Processor Name: Intel Core Duo
Processor Speed: 2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 2 MB
Memory: 1 GB
Bus Speed: 667 MHz


DVD Player:

Version: 5.0.1
Last Modified: 12/02/08 08:24
Kind: Universal
Get Info String: DVD Player 5.0.1, Copyright © 2001-2007 by Apple Inc. All Rights Reserved.
Location: /Applications/DVD Player.app

MATSHITA DVD-R UJ-857:

Firmware Revision: 4A0E
Interconnect: ATAPI
Burn Support: Yes (Apple Shipping Drive)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RW, +R, +RW
Write Strategies: CD-TAO, CD-SAO, DVD-DAO
Media:
Type: DVD-ROM
Blank: No
Erasable: No
Overwritable: No
Appendable: No

Thanks for the help,

Daniele