OK, that has eliminated the usual reasons for what you are seeing.
The next thing you need to do is make a full backup of your data in the NAS (make sure you can read the backup data), because it could stop working if it is a component failing and I include the disks as components here.
Since you only see this when you switch off your NAS and switch it on again after a short time why not leave it on 24/7/365 as we do? All our, and our clients, NAS boxes get switched off once a year when they are cleaned out inside. In the years we, and our clients, have been using D-Link NAS boxes we have seen power supplies that have died and a couple of disks that didn't pass the full SMART test, other than that no problems.