Sorry

No problem, no need to say Sorry :-)

As far as I have understood this plugin

Sorry, I didn’t notice the word “plugin” and version in the title, and was just thinking “AutoCodepage” was a translation of the translation for the “autodetect encoding” preference. Sorry for not correctly interpreting the question.

Unfortunately, no algorithm for detecting code page can work 100% of the time.

As far as I have understood this plugin, AutoCodepage is not used for detecting a code page but for opening a file with a specific extension in a specific code page.

This is set up in the AutoCodepage settings, e.g.
Group name: OEM 850 files
New filename extension(s): .OEM850
Code page to set: OEM 850
click + below the Group name panel
click + below the filename extension(s) panel

This works properly for me and OEM 850 is shown in the status bar if I open a file with filename extension .oem850

Do you have a tip for me as to where the problem lies?

Unfortunately, no algorithm for detecting code page can work 100% of the time. That’s one of the reasons that Unicode encodings are so much better than the 80’s code pages. chcp 65001 is my friend (though in your case, I have never tried it in combination with a redirected dir output in a batch, so I don’t know for sure if it will honor the chcp encodings in that case)

If Notepad++ had a command-line option for specifying the code page for the file you are opening, you could set it using the command line… But, alas, it does not. It’s probably worth a feature request (see “FAQ: Feature Request”)