I'm getting the error "could not lock RRD"

Hi I've just installed hobbit 4.2.0 on an archlinux box with rrdtool version 1.2.23-1. Looking at the logs I'm getting the following error every 30 seconds or so as it tries (I assume) to create the graphs:

2007-06-22 19:24:55 RRD error updating /var/sites/hobbit/non-secure//data/rrd/xxx/procs.rrd from 192.168.0.2: could not lock RRD

I've googled and searched the archives but can't find any other mention of this error. Can anyone suggest a solution?

Thanks

Robin