I'm repeatedly failing to download a 2.3gB demo myself (Slay the Princess). It gets to 1.5ish gB, and fails with a 403 Forbidden. Tried with Chrome. Tried with Edge. Poked around a bit ("itch.io download failed forbidden" on google) and it seems to have been happening to various people for various downloads over the last five years or so, and the workaround always ends up being "get our app, you can resume downloads from there"
It would seem a security measure (making download links unique and temporary) cuts people off on big downloads too early or something. I thought I was doing pretty good at 4mB/s, but I guess I'm not getting it done fast enough :(
Anyway, I hate installing new stuff on my pc so I'm off to find an alternative download for my thing. Getting the app will only be a very last resort, and if I have to get it, I'll uninstall it immediately afterward