I’m guessing permissions if everything (sonarr and download client) is running as a native package on the syno.
Do both run as the same user, or do they belong to the same group? Does that user or group have access to the path?
Check the entire path starting from volume1 and see if sonarr can access it.
You can do a quick test from the gui, go to manual import and try to browse to the path. It won’t tell you why, but you will see at which level in the directory structure it’s failing.