I've tried to install a new OTF font with kfontinst like system font. It asks for the root password but after it says that it is updating the font cache but it does nothing, only shows the progress window forever. It doesn't respond to press cancel button or close window. Reproducible: Always
This the command: $ kfontinst fuentes/nerd-fonts/Fantasque\ Sans\ Mono\ Regular\ Nerd\ Font\ Complete.otf Service org.kde.fontinst not registered, starting "/usr/lib64/libexec/kauth/fontinst"
Juan, is it possible you miscategorized the product and component? Font installation is handled by systemsettings/kcm_fontinst, not kfontview. Bug 361960 is a (later) bug filed for the correct product and component.
*** This bug has been marked as a duplicate of bug 361960 ***