Just a suggestion here. I remember some EARLY dual-layer DVDs eventually got some sort of layer "rot" that would cause players to not be able to read past the first layer. I've read a lot of people contacted the studio and had their DVDs replaced for free (probably also gave the studio a chance to study the issue to keep it from happening later).
It may be worth mentioning your problem to the studio in this case as well, as maybe some of the HD-DVDs have similar issues as the old dual-layer DVDs. It's completely plausable that they would replace your HD-DVD with a Blu-ray if requested. Can't hurt to try!