Majority of recently added tracks won't sync (error -50); previously added tracks sync fine

Question

Argh. I have a 5th Gen iPod Classic (80GB) and, until very recently, it's been functioning quite well.  Now I am getting error "'iTunes could not copy {song name} to the iPod {iPod name} because an unknown error occured (-50)." when trying to sync songs I have recently added to iTunes from several sources.  However, a few recently added tracks successfully synced.
In particular, there are numerous albums and songs recently purchased from Amazon that do not sync, although they play fine in iTunes.  In addition, a free song downloaded from Last.FM does not sync.  I also tried ripping a CD to AAC, and the majority of the songs do not sync.  (The first time I tried to sync the songs ripped from CD, only one track synced.  The second time, a second track synced as well.  36 additional tracks have failed to sync).
Note that there is nothing different about the format (codec, bitrate) of these newly added tracks.  Tracks added a month or so ago with identical specs and from the same sources were able to sync without a problem.
A free song downladed today from the iTunes store did sync, however.
I am on XP SP3 (up to date) and have tried all of the following:
* Updating iTunes to latest version (10.6.3.25)
* Uninstalling then reinstalling iTunes
* Updating firmware on my iPod (was 1.2, upgraded to 1.3)
* Performing "Restore" on iPod and freshly re-syncing all tracks
* Enabling the "Enable disk use" option on the iPod
* Updating the Windows registry as directed in this tech bulletin: <http://support.apple.com/kb/ts1539>
* Error checked the iPod using the Windows "Error-checking" utility (under Properties-->Tools) - no errors reported.
Further reference:
* "Convert higher bit rate songs to (rate) AAC" is not enabled on the iPod.
* There is still about 32GB free on the iPod after syncing all of my library except the new tracks
* Only antivirus software installed is Microsoft Security Essentials, which is up to date and reports no infections
What is going on??  Approximately 9600+ tracks that were added to iTunes before my recent difficulties (including tracks from many different sources) synced flawlessly when I performed the Restore.  The only tracks creating the problem are tracks added very recently to iTunes.
Is it possible that my iTunes library is corrupt?  Can I rebuild or repair it without losing anything?  Should I follow the steps in this tech bulletin <http://support.apple.com/kb/HT1451>?

Answer 1

Okay, I have a partial understanding of the problem, and more importantly a workaround.
I think the Amazon MP3 Downloader was indeed the culprit.  It is configured to automatically add your downloaded tracks to the iTunes library.  I think the most recent version of this application has some kind of bug and did not add the tracks to the library properly.
It is kind of a subtle bug, because the tracks were added well enough to see them and play them in iTunes.  However, I think the filepath that was added to the iTunes library was . . . messed up somehow?  In iTunes, the paths for the offending tracks looked something like this:
F:/Music/AMAZON~1/BEEGEE~1/NUMBER~1/01-MAS~1.MP3
It is still strange to me is that rebuilding the library did not fix the problem, but regardless, I found a way to get sync working:
(1) Made sure "Keep iTunes Media folder organized" was enabled under Edit --> Preferences --> Advanced Preferences
(2) Used File --> Add Folder To Library . . . to add the folder where the Amazon MP3 Downloader put my downloads
This reorganized my Music file into the standard Artist/Album structure.  However, the old, nonworking tracks were still in the library.
(3) Used File --> Display Duplicates, then sorted by "Album by Artist"
(4) Manually deleted all the nonworking duplicate entries.  (You can tell which one is the problem--when you click on a problematic entry, an exclamation point in a grey circle will appear next to the track name)
After this, everything synced perfectly, including the remaining tracks I ripped from the CD.  My file paths for the recent Amazon downloads now look like this:
F:\Music\Bee Gees\Number Ones\01 Massachusetts.mp3
Hope this helps someone else, and I hope the problem is fixed in a future software release from Amazon.