munin-cgi-graph: Fontconfig error: No writable cache directories

nginx gave a "502 Bad Gateway".

Service was not running:

# systemctl status munin-graph
× munin-graph.service - Munin FastCGI Graph
     Loaded: loaded (/etc/systemd/system/munin-graph.service; static)
     Active: failed (Result: exit-code) since Fri 2023-12-08 08:29:38 CET; 17s ago
   Duration: 531ms
TriggeredBy: × munin-graph.socket
    Process: 155856 ExecStart=/usr/lib/munin/cgi/munin-cgi-graph (code=exited, status=28)
   Main PID: 155856 (code=exited, status=28)
        CPU: 527ms

Dec 08 08:29:37 dojo systemd[1]: Started munin-graph.service - Munin FastCGI Graph.
Dec 08 08:29:38 dojo munin-cgi-graph[155856]: Fontconfig error: No writable cache directories

Solution:

1. Free space on /var partition (was full)
2. Run fc-cache --force