Picard sometimes gives odd "Permission denied" errors when trying to save files on a NAS/network share on Windows.
It fails regardless of save, save+rename, save+rename+move combinations; but the issue is saving tag changes to the file where the file is on a network path location.
The error is always like the above, and always in _insert_bytes_no_mmap,
Workaround #1: Move the files to C: first, and have Picard save+rename+move files onto NAS.
Workaround #2: Disable embedding of cover art.
I have only ever seen this error with mp4s, not mp3s, but that might be coincidence.
Test file, in case it matters: http://www.mediafire.com/?0m0jg1x5yzan9ic (all files in an album typically fail together)
It seems to have something to do with the cover art linked to the release release.
Using the old "cover art downloader" plugin
Tagging against http://musicbrainz.org/release/f5e332c9-df5e-4ad9-9a5e-f23c5be4e43a - OK (20kb first front cover art image)
Tagging against http://musicbrainz.org/release/a7c4ca53-fa6e-4918-897f-e2f9c4d2ed61 - PERMISSION DENIED (135kB first front cover art image)
With the inbuilt cover art downloading in Picard 1.2 both releases fail. I think this is because the 2nd image for the first release above is also a large image (635kB).