Cannot create checkbox in admin fields

Hi,

I’ve started using WCFF to add custom admin fields groups to my products. I have created one group called “Common”. I have successfully added select and text fields to this group but when I try to add a check box it fails and I get the following error in my browser debug consol:
https://oursite.com/wp-content/plugins/wc-fields-factory/assets/img/.png 404 (Not Found)

Any idea why this is happening? Is this a known issue?
Thanks in advance.