Connections to NAS frequently cause problems.
Your observation that MP3tag works without problem with smaller amounts of files would indicate to me that there is something happening in between MP3tag and the files.
And I would think this is the NAS and its indexing SW.
Still, you could make sure that the files are OK (and tagging then initially with MP3tag does not guarantee that they were OK in the first place), see the linked tools in this thread:
and if all that does not help, I am afraid that only treating smaller amounts of files will be the workaround.
See also here for NAS problems: