Starting out with Google Cloud Storage

Hi

I don't seem to be able to get Google cloud backup to work. I followed the instructions here to generate credentials at Google: https://www.virtualmin.com/documentation/cloudmin/vm/gce-account, and entered and validated them at Virtualmin

At Google I created a project "Virtualmin" and a bucket "xxxxxxxxxx".

However whenever I try to backup I get:

Starting backup of 1 domains to Google folder xxxxxxxxxx ..
Invalid argument.
Backup failed! See the progress output above for the reason why.

Can you advise what I'm doing wrong!

I have:
Bucket & path: xxxxxxxxxx (or xxxxxxx/test)
Full
Single file
Transfer each virtual server after it is backed up

CentOS 6.6
VM Pro