Because that isnt a Dropbox shared link, that is just the direct path to a file in somebody's own folder (i.e. they've opened the file in their own browser and copied that link rather than making a shared link).
As a common format it's a valid Dropbox link, BUT are you sure GrapplingProject.rar is still into creator's account? Once particular file gets removed from Dropbox, its link "dies" too! There is also another possibility - was the particular file ever been temporarily removed (even for a moment) from Dropbox after the link creation? Take in mind that Dropbox link point to particular content, NOT to particular file name!!! That's why when you remove and add a file (including the same), it's considered as a new content and new link have to be created! The old one gets invalid. For the same reason when you move some file inside Dropbox (or even rename it) the pointing link remain valid (the content is the same 😉).