@kovidgoyal
Ok, I was able to replicate the creation of the "data" folder on my desktop when adding a new library entry. I can't seem to replicate the error message from my first post.
I captured the process in a video. That will show the exact steps I have taken. You can probably skip forward while I modify the metadata fields but it shows exactly what I am changing.
It also seems that it happens right after starting Calibre and adding the first new library entry. It doesn't happen when adding a second library entry.
Note at the start on the left margin of the desktop there is no \data\ folder, then at the end there is.
mkv files aren't accepted for upload to the site, so I have uploaded it here...
https://easyupload.io/ggh2ka