Checking around the community, that 404 error implies that the asset was migrated from the old marketplace to FAB. Your asset should still be available, it just needs to be re-downloaded from scratch.
To do so, go to FAB via web browser, and check your library to look for the asset pack. Once located, add it to an empty project, which should force the re-donwload. If the issue persists, I would suggest clearing out the contents of your VaultCache folder in your Epic folder, then try the download once again.