After backup: Destination is read-only?

StephenB wrote:
The most likely cause of that error is that the "dirty" bit is set on the disk volume. Windows scan/fix should reset that. The dirty bit is often set when the disk is not ejected before removal, or if the NAS sees some other issue when mounting it. I back up over the network, and not to local disks - so I don't have direct experience on how often this occurs.

I have adopted a strict "always eject first" policy ever since USB external disks and memory sticks became available and use the same approach when connecting via USB directly on the NAS. I use the Eject button always. So that can't be it.
I do a chkdsk on Windows first to make sure there are no problems with the drive. Windows tells me that the disk is OK. If the NAS sees a problem when I connect it there, there should at least be some hint on what is wrong in the logs.
I tried backing up via the network also, but with > 4 TB that takes forever and the network speed seems to decline the longer into the backup I get. So for me that's not an option.
So this brings me back to my initial point: "This makes me think it is either a problem with my data on the NAS or a problem in the FrontView backup procedure". But still not closer to finding out what is at the hart of this problem.

Has anyone found a root cause for the "Destination is read-only" message?
Is there any way that I can get more information on what went wrong during the backup (e.g. what file is causing the message)?