Error: unexpected end of file at offset....

[mars51]

Hi all, Newbie here. I thought this might relate to tagging, if not please move it appropriately.
Despite all the misguided comments in other places that Minimserver is hard to set up, I followed Simon's guides and had it working & running in an hour. Great stuff.
Minimserver is running on a QNAP TS-412 and will be connecting to a coming Lumin, replacing a Mac. In the meantime, it's a library in JRiver. I have some expected tagging issues but those I can sort, I hope, but the main issue I need help on is 3500 missing files. Several thousand of these appear as 'Error: unexpected end of file at offset XXXXXXX' in the Minimwatch log. Most are .flac, some are .aiff. I can solve the problem of missing .aiff by converting them to .flac but it will take weeks.
A search didn't throw anything up. Grateful for any help on this.