Having installed Munin recently on my servers, I started to get emails from cron saying
Fatal error: Could not open "/var/lib/munin/datafile" for writing: Permission denied at /usr/lib/perl5/vendor_perl/5.8.8/Munin.pm line 727. (process:18039): Pango-WARNING **: Invalid UTF-8 string passed to pango_layout_set_text()
I also got some errors about /var/log/munin/munin-update.log being inaccessible. Somehow these files does not belong to munin:munin, but to rrdcache:rrdcache, so the solution is just to make them chmod 0777 and that should solve the problem.