This fix cleans a bit too much. In /usr/lib/munin/cgi/munin-cgi-graph the string "/munin-cgi-graph" is appended to $config->{cgitmpdir}. The cronjob that cleans cgitmpdir removes that directory as well even if it needs to stay, since it won't be created automatically.
The fixes I've found is either recreate the directory with: install -o www-data -g munin -m 0775 -d /var/lib/munin/cgi-tmp/munin-cgi-graph/ Or change the cronjob to find with mindepth of 2 instead of 1. -- Klas