I've also been having this exact issue with my Synology DS916+ running DSM 6.1.3-15152 Update 8. I've had my NAS for about a year, however my imports have been failing about 60% of the time for the same reason for the last couple months.
Manual import is successful every time. I have verified permissions, folders and files are all 777. I only have one user on my system "admin".
I have tried various installs/uninstalls using Sonarr & Nzbget in various forms, in Docker, from Synology, from Synocommunity. I have even re-installed directly via shell.
One thing I've noticed is all failed imports are obfuscated mkv files. Might just be a coincidence - I use Nzbgeek and NZBfinder.
Strange that it it does not fail 100% of the time.
Perhaps the developer could consider adding an option to bypass a mediainfo sample check, and just automatically move/import files > specified size. For example, a 1.5gb .mkv file is probably not a sample.
Sonarr Version 18.104.22.16854
Mono Version 4.6.2
NZBGet 20.0-r2159, Umask 000