Applicant getting error code during submission

I have a member of our association calling with an error that says "Your file was not found. It may have been moved or deleted." 

I'm assuming its an error on his side- but is this something you've seen before? Any thoughts on how he could fix it?

When is that error message shown? When the user tries to submit the form?

Please provide a screenshot of the error message.

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Also provide us a link of your form or the website URL if the form is embedded, so that we can test it.