Hello and welcome!
Which Duplicati docker image are you using? The official duplicati one, or the linuxserver one? The linuxserver one doesn’t run Duplicati as root by default, so access issues are much more common.
In your docker config for this container, where are you mapping /backups ? Can you double check permissions on that folder from your normal NAS interface? If the error really does only happen at the end of the job, maybe it means it was able to write to /backups but not read from it. (Duplicati does a verification test at the end by reading three files and checking them.) Do you think you may have set the permissions to write-only on that folder?