maybe I am misunderstanding your question, but you should still be able to open the proof. Even if the page shows a load error, I thought the comments should still show up beside the error. And then clicking on a comment would show you a screenshot of what the page used to look like. If that's not the case, then if the proof was being stored in a Workfront object like a task or project, you should be able to navigate to where the link is being stored, and then click on the details link of the object in order to see the updates (comments).
