I got here because of an old Q&A from weed that came up on Google (https://www.experts-exchange.com/macintosh/Q_11372297.html
I am receiving the same Hardware missing message (21) from Apple DVD Player on my blue + white G3. 48 hours ago I could watch DVDs on my G3. I just updated to OS9.1 from OS9.0.4 using a Powerbook G4 Install CD (which I was told by guru roommate would have no conflicts). I have since bounced around from OS8.5.1 to 9.0.4 to 9.1 again, and worked with Quicktime versions 3.x, 4.12, 5.0.2, and now 6.0. Even back in near original configurations it won't play a DVD, it does recognize it.
Everything else is working great (after 24 hours of extension replacement). Apple System Profiler even says that the decoder hardware and CD-DVD ROM drive are in place, so I'm hoping the decoder didn't get fried.
Can I blame the roommate/powerbook thing or is there an insidious extention causing a hardware error message?