I have emailed support but as I hadn't heard back from that address in relation to previous queries I am posting here too to ensure you are aware of our issue.
We are using WordPress 3.5 (the site relies on plugins that won't work on newer versions) and version 1.8.1 of your plugin together with the zip and email premium extensions.
This has worked fine on another site but we now have an issue where the 208mb zip file is considered 'invalid' by Windows. Reducing the size of the backup file by excluding wp-content to around 57mb results in a zip file that is valid and can be restored.
I have sent more information via email and prefer to deal with the support call in that way but wanted to be sure you were aware of the issue.
Thanks in advance for your help.