Errors in scanner.log

I've noticed that recently my scheduled scan at 1am isn't completing and so
yesterday I decided to do a complete rescan of my collection. It goes OK up
until the point where it pre-caches the album artwork then this:

14-04-16 18:02:36.6655] Slim::Music::Artwork::precacheAllArtwork (447)
Starting precacheArtwork for 2451 albums
[14-04-16 18:02:49.6615] Slim::Schema::Storage::throw_exception (122) Error:
DBI Exception: DBD::SQLite::st execute failed: database is locked [for
Statement "
UPDATE tracks
SET coverid = ?, cover_cached = 1
WHERE album = ?
AND cover = ?
"]
[14-04-16 18:02:49.6619] Slim::Schema::Storage::throw_exception (122)
Backtrace:

frame 0: Slim::Utils::Log::logBacktrace
(/usr/share/perl5/Slim/Schema/Storage.pm line 122)
frame 1: Slim::Schema::Storage::throw_exception
(/usr/share/squeezeboxserver/CPAN/DBIx/Class/Storage/DBI.pm line 1006)
frame 2: DBIx::Class::Storage::DBI::__ANON__
(/usr/share/perl5/Slim/Music/Artwork.pm line 514)
frame 3: Slim::Music::Artwork::__ANON__
(/usr/share/perl5/Slim/Utils/ImageResizer.pm line 134)
frame 4: Slim::Utils::ImageResizer::sync_resize
(/usr/share/perl5/Slim/Utils/ImageResizer.pm line 108)
frame 5: Slim::Utils::ImageResizer::resize
(/usr/share/perl5/Slim/Music/Artwork.pm line 541)
frame 6: Slim::Music::Artwork::__ANON__
(/usr/share/perl5/Slim/Music/Artwork.pm line 561)
frame 7: Slim::Music::Artwork::precacheAllArtwork
(/usr/share/perl5/Slim/Music/Import.pm line 397)
frame 8: Slim::Music::Import::runScanPostProcessing
(/usr/sbin/squeezeboxserver-scanner line 333)
frame 9: (eval) (/usr/sbin/squeezeboxserver-scanner line 333)
frame 10: main::main (/usr/sbin/squeezeboxserver-scanner line 494)

[14-04-16 18:02:49.6657] main::main (337) Error: Failed when running scan
post-process: [Carp::Clan::__ANON__(): DBI Exception: DBD::SQLite::st execute
failed: database is locked [for Statement "
UPDATE tracks
SET coverid = ?, cover_cached = 1
WHERE album = ?
AND cover = ?
"] at /usr/share/perl5/Slim/Schema/Storage.pm line 126
]
[14-04-16 18:02:49.6661] main::main (338) Error: Not updating lastRescanTime!

Any ideas what is causing this?