Another topic might be better if this post doesn't help. This one is marked resolved by the poster.
The status sometimes goes to 100% in your browser even though it isn't downloaded. Just check the size of the downloaded file to make sure it is 495MB. If it is, raise another topic we can investigate. If it isn't then the issue is the file download, which is getting resolved shortly - we're nearly finished with the changes needed.
In the meantime - not ideal I know - but you could install a File Manager plugin for WordPress if you can find one, and go to "wp-content/plugins/wponlinebackup/tmp/". Should be "backup.zip.php". Rename this to "backup.zip", then download it directly. It will be scrambled slightly because it was directly downloaded, but if you use 7-Zip to extract it, it might be able to repair the archive if you use repair option.