Basically, Duplicati can only back up a working drive. Network drives are more likely to break.
sounds similar, and has a comment from the lead author that the drive is expected to work right.
If Mountain Duck support has some debug ideas beyond you running Process Monitor, try them.
You can try The TEST-FILTERS command in Duplicati to walk a folder without all the other stuff. Knowing whether simplifying things that way helps (or not) might be helpful to debug the drive…