So, I was trying to download the Short Story collection for Amnesia, but whenever I click the download link, it gives me a white page saying "403 - Forbidden." At first, I thought it was just something wrong with Amnesia's files, so I tried to download The Cat Lady wallpapers and received the same error. Why am I getting this error, and how do I resolve it?

Are you using the GOG downloader? If so, are you using the newest version?

Try (re)downloading the entire bundle, including the game, even though you've (presumably) d/l'ed the game itself. The 403 error may arise because you're trying to grab just part of the package. (You can always delete the extras you didn't want off your hard drive.)