After fresh installation «This operation is not allowed»

I have just sucessfully installed Nextcloud No error messages so far. After my first login, I am in the «All files» section. There are 4 jpg images (thumbnails and filenames). On top of the window, there is an error message «Diese Operation ist nicht erlaubt» («This Operation is not allowed»). Clicking these images multiplies this error message.

Then I go to «Neueste» (Newest). There are the 4 images mentionned above and furthermore 1 png image, 2 pdf files, 1 txt file, 1 odt file and 1 mp4 file.

Any file operation is possible except for the 4 jpg files. Example: deleting the file Nut.jpg displays the error message «Fehler beim Löschen der Datei Nut.jpg» («Error deleting the file Nut.jpg»). Then the page is refreshed automatically.

Deleting the txt file is successful, but the filename remains visible. Any additional file operation displays the error message «Error deleting the file… since this file does not exist any more».