Well, the problem is is that I tried that.
Assuming that I was somehow using the old catalog 1.0 link, I went into Workshop -> Settings and deleted the link, but when I went into the C:\Users\username\AppData\Roaming\7thWorkshop folder, there was no Catalog.xml to delete. So I closed and then reopened 7H and clicked Workshop -> Check subscriptions now, but nothing popped up. So I went to the Catalog tab, hit "search" and the modlist populated, but the same error happened when I tried to download a mod. Also, I noticed that there's still no Catalog.xml file in the C:\Users\username\AppData\Roaming\7thWorkshop folder.