Failed backup but why?

I don’t know what you mean by a shell. Are you saying it doesn’t actually access remote files?
From what I can find, you have at least the option to not use Windows protocols, but run NCP.
That’s how access gets done, right? It’s not like there’s SMB or CIFS that it’s “just” extending?
OES 2023: NCP Server for Linux Administration Guide
NetWare Core Protocols Documentation

This is why I suggested testing a single file to be nice and easy. Can it handle that file?

If you mean CIFS the SMB dialect with direct Windows support, you could certainly test:

Microsoft SMB Protocol and CIFS Protocol Overview

I’m glad you know that. You could probably get PowerShell confused then, as it’s .NET.
If there’s a problem at the .NET level, that’s below Duplicati, so maybe can’t be solved.

Neither the size nor the file counts seem high. I’m not sure where you get 500 warnings.
That’s less than all the files, so maybe there’s some pattern, or just temporary overload.
Either getting a log file and looking, or testing different areas and sizes might be helpful.

The screenshot does explain how OES appears to be an extension, but I think the tabs
that don’t say OES are actually provided by NCP, though tab doesn’t specifically say so.
Windows has some sort of adaptability where its view changes based on what’s around.

Channel Pipeline is complex, but the 20 warnings in the default log are not opening files.
Enumeration is primarily just a walk of the source trees, so I’m not sure how that can fail.
One possible guess is that the file attribute filter got upset, but there are few clues so far.
Process Monitor might have some, and (less likely) a stack trace might give a little more.