The RRD ERROR lines are indicating the graphs (pngs) can't be created because something in RRD is unable to find (or open) some file. This snippet is saying it's trying to open a blank path, and failing.
opening '': No such file or directory
The perl error mentions an "uninitialized value $filename", which given the RRD error is probably part of the problem. Since the graphs are generated out of the RRD files, the likely issue is that the system can't find your RRDs. Consequently, you'll want to check that everything about specifying the location of your RRDs is in order.
- Confirm the dbdir directive in munin.conf is correct
- Confirm your RRDs are present in the $dbdir/domain/ path
- Confirm filesystem permissions allow the munin process to read and write said path
- Confirm the host tree (in munin.conf) is correct for the client you're having trouble with
Regarding triggering graph creation: the munin server is driven by cron. You should be able to find a cron job which you can manually run as the munin user in order to trigger the process. The RPM I've used in the past has this job at /etc/cron.d/munin