Failed to import episode, Destination already exists error

Sonarr version (exact version): 4.0.9.2244
Mono version (if Sonarr is not running on Windows):
OS: Synocommunity
Debug logs:
Description of issue: I’ve been maintaining a home Synology NAS that runs all apps such as Plex, Sonarr, Radarr, Overseerr, Nzbget, etc. Lately both Sonarr and Radarr give me errors when downloads are completed. Basically Nzbget downloads to the folder /volume1/NzbgetDownloads/TV Shows and then Sonarr picks it up, places it in /volume1/Downloads/TV Shows. But then Sonarr leaves the downloaded file in the folder /volume1/NzbgetDownloads/TV Shows, shows an error under Activity-Queue - Failed to import episode, destination already exists.

I have checked Linux permissions multiple times, attached permission screenshot. Funny thing is that whenever I go into permissions, edit them to the same settings, the issue gets fixed for the time being. But when the next download happens, the issue is back.

I’ve check the logs, shows “System.UnauthorizedAccessException: Access to the path ‘/volume1/NzbGetDownloads/TV Shows/Kaos.S01E03.1080p.WEB.H264-ETHEL/GWg8ciVk7uHPCZjrKbFU.mkv’ is denied.
—> System.IO.IOException: Permission denied”.

Any help would be much appreciated.