Error running omv-mkgraph

I've been trying out 3.x and so far, a bit rough around the edges but very usable.

I do have an error with RRD graphs - when I run omv-mkgraph this is the output:

ERROR: rrdc_flush (/var/lib/rrdcached/db/localhost/cpu-0/cpu-idle.rrd) failed with status -1.

I notice none of the files are there inside /var/lib/rrdcached/db, so I created this one by hand but still no go.

What can I do to fix this? Thanks!