That did the trick, thank you.
Reported this behaviour as a bug, can be found at
https://bugs.launchpad.net/calibre/+bug/1698441
For the time being, following your advice I created a .desktop file to launch Calibre at "~/.local/share/applications/calibre-gui.desktop" with the following contents:
Code:
[Desktop Entry]
Version=3.0
Type=Application
Name=calibre
GenericName=E-book library management
Comment=E-book library management: Convert, view, share, catalogue all your e-books
TryExec=calibre
Exec=env CALIBRE_CACHE_DIRECTORY='/home/YOURUSERNAME/ROUTETONEWCACHEFOLDER' calibre --detach %F
Icon=calibre-gui
Categories=Office;
MimeType=text/fb2+xml;application/x-cbz;application/vnd.openxmlformats-officedocument.wordprocessingml.document;text/html;application/x-mobipocket-ebook;application/ereader;application/x-mobipocket-subscription;application/x-mobi8-ebook;text/rtf;application/x-cbr;application/vnd.ms-word.document.macroenabled.12;application/xhtml+xml;application/x-sony-bbeb;application/oebps-package+xml;application/pdf;application/x-cbc;application/vnd.oasis.opendocument.text;text/plain;image/vnd.djvu;text/x-markdown;application/epub+zip;
After launching Calibre from this new .desktop file it will take precedence in Ubuntu's launcher over "/usr/share/applications/calibre-gui.desktop" and will be set automatically as the default launcher.