This can be Wordfence plugin blocked the operation when you uploaded the file, in that case, the best thing to try would be Learning Mode and try the backup process again.
You can switch to Learning Mode via Wordfence > Firewall > All Firewall Options > Web Application Firewall Status and switch from Enabled and Protecting to Learning Mode. Try the action you were attempting to perform again, and then head back over afterward and switch back to Enabled and Protecting. If the action was successful, it will be whitelisted.
If it does not work, try to use FTP to upload the backup to the wp-content/wpvividbackups directory of the site and scan uploaded backup to display it in backup list.