The problem is solved.
The two files seem to be exactly the same in two directories (admin and user). At some point the error display changed from “admin” to “user”. Since everything else was the same, I didn’t notice. Evil trap!
Next, I moved the additional directory somewhere else, ran a new scan and moved the directory back. Now there is no more error message. Many thanks to everyone who helped me 
Should one generally copy the entire Nextcloud archive to the server in binary mode in the future? And because this is my first Nextcloud I am interested in another question: How is an update done?