Can you post your policy.xml file?
Sometimes there are different policy.xml files in different directories /etc/ImageMagick-6/policy.xml for ImageMagick version 6, /etc/ImageMagick-7/policy.xml for ImageMagick version 7, etc. From your log file, your AMC installation uses ImageMagick version 6.

Here's the policy.xml file from directory: /Applications/MAMP/Library/etc/ImageMagick-6 on a Mac.

Thanks. It seems that this policy.xml is OK.
Maybe some security app on your computer like F-Secure or equivalent blocks AMC accessing the scan files?
Or these files are not readable by the user that runs AMC?

The files seems to be accessed fine. They are shown in the "Unrecognized scans" list.

Can you try these commands from a terminal?

identify -version
identify -list policy
identify /home/allgemeine/AllgI_20181002/IT041251_Canon115_RPS_2095_001.pdf

I couldn't find any "allgemeine" directory on my computer.

The directory /home/allgemeine should exist, since it appears in the log file.
From your log file:

identify: unable to load module `/opt/local/Lib/ImageMagick-6.9.0//modules-Q16/coders/pdf.la': file not found @ error/module.c/OpenModule/1282.
identify: no decode delegate for this image format `PDF' @ error/constitute.c/ReadImage/501.

Ok, I'm very sorry, the log file I sent at the beginning was completely bogus (not sure where I got this from).

I've now turned on debug information and got the one attached.

OK. Can you open the unrecognized scans window, select a scan, click on "preprocess" (the gears), and post a screenshot?

A screenshot of what? The unrecognized scans window just shows the scan that I uploaded.

Attached are the messages that appear in the terminal when I press on the gears symbol.

How did you install AMC?

Following the instructions on https://github.com/maelvls/homebrew-amc

I don't know homegrew but maybe you can try to upgrade or reinstall opencv and AMC with some of these commands

brew upgrade
brew cleanup
brew reinstall opencv
brew reinstall auto-multiple-choice

I reinstalled the files as you suggested and now it works!

Thank you so much for your help!

Great. I don't know what exactly went wrong, maybe some version mismatch for some weird reason…