Rsync and Virtualbox backup

I’m having a strange problem backing up my /home folder with rsync. Everything works fine but the Virtualbox hard disk in the .virtualbox folder is backed up only on alternate runs: i.e. it works on day, next day the hard disk is missing, following day it’s back again. Rsync error concerning the .vmdk file is “File exists (17)”.
I’ve googled a lot but can’t find any answers. This happens whether the VM is running during the backup or not. Any thoughts?

Solved, by using the --inplace rsync option.