How to debug “Fontconfig error: No writable cache directories” as the console output of font-manager?

Starting

font-manager

as a regular user from a console (regardless of the presence or the contents of /usr/local/share/fonts) prints four messages

Fontconfig error: No writable cache directories

on the console. /var/log/syslog and dmesg show nothing interesting, and running

fc-cache -r && fc-cache -fv && dpkg-reconfigure fontconfig fontconfig-cache font-manager

as root didn't help.

How to debug this error?