If I – using the online interface – choose “upload file”, and then select a file that already exists in my target folder, Nextcloud displays the correct response:
Found the error: there is an extra space in one of the two file names.
the old one: Presentation The Reef[space].pdf
the new one: Presentation The Reef.pdf
Solution:
move the new file to a different folder.
on the “right” folder (Key documents), rename the old version deleting the space. I had to do this with Finder on my machine, when I tried it through the web interface it did not work.
re-upload the new version using the web interface.