Errore nella creazione della miniatura: convert-im6.q16: delegate failed `'rsvg-convert' -o '%o' '%i'' @ error/delegate.c/InvokeDelegate/1928. convert-im6.q16: unable to open file `/tmp/magick-5789c1a0JSf8Ufii': No such file or directory @ error/constitute.c/ReadImage/600. convert-im6.q16: no images defined `PNG:/home/www-data/vhosts/informazioni.wiki/tmp/transform_0ec1d3e74f96.png' @ error/convert.c/ConvertImageCommand/3258.
It seems that the error message was misleading and the error was not about the file but, instead, related to the nonexistence of the rsvg-convert command.
In Debian GNU/Linux buster (current stable) the command /usr/bin/rsvg-convert is provided by the librsvg2-bin package.
I've fixed with:
apt install librsvg2-bin