I've not found a way of discovering what they think the error is.
A few times I have taken a screenshot of my application, so I can check what I've requested, but frankly I usually can't be bothered!
When I had rejections where I had used a single forename, I remembered wondering on filling in the form whether they really meant 'name' in the singular, but decided that if that's what they asked for, they might reject an application with two or more (okay, it sounds as if I overthought that!). I tried one application again, this time with all the forenames, and got the document. I'm pretty confident therefore that the single forename was the issue. I can't prove it of course!