There are many warning on "xxxx login Fail (SSH)"
Applied models: All NAS Series
Applied Firmware: All QTS Firmware
This is due to NAS is under attack from unknown programs/persons and please don’t forget to update firmware to latest, update malware remover app on App center, and change all users password if passwords are too weak. Other options you could change to improve security, you could change the port of SSH service, or stop forwarding the port to the internet. You could also access QTS system via https instead of http (port 8080) if you need to connect from the internet.
Please also refer to the following article What is the best practice for enhancing NAS security?