Just a hunch. Are there any options in your backup software that are possibly holding the connection / share to the NAS open after it completes its thing?
I will have to go through the back up software bit by bit, and see. I also use allsync to sync my documents folder to the nas as well, not sure if that is causing the problem, it does a sync when files are changed, maybe I should set that to do it at a certain time or when the computer is turned on.
I wonder if I should use the I.P address of the nas instead.
I also had the error when using Windows, but that seemed to have sorted itself out now.
Strange that a friend of mine never gets these errors with the network I helped her with,

She is using fibre, but I doubt that make s a difference.
Adrian
Desktop machine Ryzen powered with windows 10 , reluctantly.
Plusnet FTTC