Fortunately it is not the NAS but most likely a bug in macOS 26 Tahoe.
The volume name of the no longer working backup sparsebundle is “Reservekopieën van Computername”.
The volume name of the newly created backup sparsebundle under Tahoe is: “Back‑ups van Computername”
This is the hint in the log(1) output that pointed me to the issue being the volume name:
backupd-helper: (TimeMachine) [com.apple.TimeMachine:MountPointValidity] Invalid mountpoint '/Volumes/Reservekopieën van Computername' - no volume mounted at this path
After changing the volume name of the no longer working backup to “Back‑ups van Computername” and re-adding it to Time Machine, the backup is operational again.
I am not certain if the localisation change from “Reservekopieën” to “Back‑ups” in Tahoe is the cause, or that Tahoe does not like the “ë” in the name of the backup volume name.
Glad we have our backup histories back, and hopefully this will help others with the same issue.