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Eine weitere mögliche Ursache für nicht angezeigte Bilder kann der Dateiname sein:
background-image:url("files/Interface/_DSC4992-2%20frei_small.png"); funktionierte nicht...

Yes, I already had the file, but to go back to the provider I had to ascertain that it was "THE" official contao file.
It turned out that somehow a nonprinting character had got into the text which...

following https://community.contao.org/en/showthread.php?24437-Contao-Manager-skips-installation-step item #6 ("Try to add a new web/.htaccess file with this content: "), I've added the proposed...