iTunes importer skipping albums as "File not found"

If I just use the iTunes importer in 7.9 (on a Synology NAS) to import my music, it gets *almost* all of it, except for a few things where it says errors like

[16-03-08 21:37:56.8019] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/02 Next Summer.m4a"
[16-03-08 21:37:56.8135] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/03 These Rituals of Mine.m4a"
[16-03-08 21:37:56.8248] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/05 Under the Moon.m4a"
[16-03-08 21:37:56.8364] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/04 Action_Reaction.m4a"
[16-03-08 21:37:56.8477] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/06 Wintertime Love.m4a"
[16-03-08 21:37:56.8591] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/08 Why Must It Always Be the Same.m4a"
[16-03-08 21:37:56.8702] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/07 She Walks.m4a"
[16-03-08 21:37:56.8812] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/09 Claustrophobia.m4a"
[16-03-08 21:37:56.8924] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/01 Hollow Talk.m4a"
[16-03-08 21:37:56.9034] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/10 Yamagata.m4a"
[16-03-08 21:37:56.9146] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/01 Hollow Talk.mp3"
[16-03-08 21:37:56.9260] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/02 Next Summer.mp3"
[16-03-08 21:37:56.9376] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/03 These Rituals of Mine.mp3"
[16-03-08 21:37:56.9493] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/04 Action_Reaction.mp3"
[16-03-08 21:37:56.9607] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/05 Under the Moon.mp3"
[16-03-08 21:37:56.9718] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/06 Wintertime Love.mp3"
[16-03-08 21:37:56.9830] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/07 She Walks.mp3"
[16-03-08 21:37:56.9942] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/08 Why Must It Always Be the Same.mp3"
[16-03-08 21:37:57.0058] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/09 Claustrophobia.mp3"
[16-03-08 21:37:57.0169] Slim::Plugin::iTunes::Importer::handleTrack (342) File not found: "/Users/sysjkf/Music/iTunes/iTunes Media/Music/Choir of Young Believers/This Is For the White In Your Eyes/10 Yamagata.mp3"

But this is *exactly* the same directory structure as the rest of the files that it finds, no problem. I've applied all the same permissions on the Synology NAS to all the subdirectories and files of my iTunes Media directory, and I've told LMS the location of my iTunes Media directory. As I say, it finds 99% of the 10,000 tracks in my library just fine, and there's no obvious difference in my iTunes Library.xml (created with "Export Library" from the latest iTunes for the Mac) between this directory and any other.

So why can't it find these files, when it can find all the others? I've tried Googling for the error message, but that doesn't find any answers.

If I switch off the iTunes importer and set the Music folder in LMS and import that way, it finds all the tracks just fine. So it can't be a permissions problem. But I want my iTunes playlists, and I don't want all my tracks imported twice.

Any ideas would be much appreciated, this is driving me nuts. 99% of my music imports just fine.

Thanks all!