Some WMA/DRM files won't sync after firmware upgrade on Zen Touch

Question

I downloaded an eaudio book from my library, which grants a 2 day license. Prior to upgrading the firmware (to the latest available) on my Zen Touch, I was able to upload this book and play it while walking, etc.
At the same time, I was able to upload several other books I had downloaded and licensed. All are WMA/DRM.
After upgrading the firmware, I was able to resync some of the books I'd downloaded to my Zen Touch. Several, however, yield error messages--no matter how I try to upload them (WMP, Creative MediaSource, Zen Touch Media Explorer). All refuse to let me upload.
These books previously uploaded/played just fine. Note: they still play fine using WMP and Creative's software. But, it's a bit difficult to lug my desktop around when walking/running through the woods.
Any idea what the problem might be? All MediaSource says is:
<IMG src="http://herb.thetysons.org/images/CMC-error.jpg">
WMP0 tells me that "an unknown error" has occurred. Neither event is significant to make it to the system's application event log.
Any ideas on why these files--which could be tranferred before--can't be transferred now? The licenses are still valid, and the file format is identical to those that transferred without incident.

Answer 1

OK, my update. After reading the recent responses (thanks!), I performed the following experiments.
1) Reverted to the original v2.0 via factory restore, relinked to the Mac. ATV immediately copied over my entire library (one movie, lots of music) from the Mac, because I have iTunes checked marked for Automatic Sync ("iTunes automatically chooses the audio and video to sync, etc.").
*I consider this to be normal and expected behaviour.*
2) Downloaded and installed the 2.1 update (again...). After reboot the ATV was still on my network and still linked to my Mac. However there was no longer any media content on the ATV (as before).
*I do not consider this to be normal, expected behaviour. Nor, for that matter, is wiping user content acceptable.*
3) Cancelled and relinked ATV to Mac. As soon as the linking code was entered, automatic sync started. Only the one movie was copied, but no music.
*I do not consider this to be normal, expected behaviour.*
4) When that sync was completed I went and checked all the songs in one album in iTunes as a quick test, then sync'd again. That song was promptly copied to the ATV.
Progress!
5) Checked all songs, initiated a new sync and they're being copied as I write.
Success!
So v2.1 requires music to be manually selected in iTunes in order for automatic sync to work. Anyone not spot a contradiction in terms? Furthermore that was certainly not the case in software versions prior to 2.1, nor was this change communicated in any way I'm aware of to the user.
My conclusion is that this is either a bug, or else sheer thoughtlessness on Apple's part.
But my sincere thanks to those who've helped me to restore proper function to the ATV (well, fingers crossed), a device that I've become very fond of.
Neil