error with omv-mkgraph

hi there when running

omv-mkgraph

it returns me:

ERROR: rrdc_flush (/var/lib/rrdcached/db//localhost/sensors-coretemp-isa-0000/temperature-temp2.rrd) failed with status -1.

Yes I installed a while ago the omv-sensors thingy, and there must be some code/reference left somewhere. Where can I search?

The graphs (RAM, load, nic, cpu, disk) are displaying correclty, but sometimes I receive a 'software failure' and need to login again. then I need to wait till the next graphs are generated. That's why I was investigating omv-mkgraph.

Thanks for your input.