We got the file, and we can see the corruption - all the data is there but the corruption is odd we're not sure how it's happened.
We've got a diagnostics version you can try which will generate a log.
You can download it here: https://wordpress.backup-technology.com/wponlinebackup-423-tracing.zip
You'll need to Deactivate and Delete the plugin you've already got installed (please note it will lose your settings, sorry). Then you go to Plugins, then click Add New at the top, click the Upload option and then upload the ZIP file above.
It will look like the usual plugin but will create a file called trace.log at /wp-content/plugins/wponlinebackup-423-tracing/trace.log
Please run an online backup and reproduce the problem. The log will have some file names and paths inside but nothing any more sensitive than that.
You can email your blog URL to support "at" backup-technology.co.uk and we can try to access the trace.log in our web browser and see what's happening. Or you could download it via FTP, compress it, and email it.
We can then investigate. Let me know if you aren't sure on any of the above.