For several months I've been using the instructions in this blog post to use Workbench to backup the metadata in my org. Last month, the process stopped working. I assumed it was a one-off oddity and didn't think much of it, but I'm seeing the same errors again this month. Generating the XML "package" containing the metadata I want to export works fine and the process appears to succeed. However when I go to click the download link in Workbench I am told the .zip archive no longer exists. I've uploaded screenshots to show that the ZIP file is generated and the error message displayed when I click the download button. 

I have no idea what I'm doing wrong, and don't believe I've changed anything compared to what I was doing previously, when the backup worked. 

I know there are alternative methods for doing this, such as those discussed here https://developer.salesforce.com/forums/?id=906F0000000BIYzIAO but I am interested in understanding why the method I've been using ceased to work.