With a shared project (not the owner), the page opens fine in the web browser but gives an error 404 when you try to open it in the app (macOS) with the same account it was shared to. I can see the project just fine, but it can’t be opened.
You can see shared projects in the left panel in the main menu (under all your teams).
I think you misread my problem. I know where to find shared products that’s not the issue, I literally wrote: “I can see the project just fine, but it can’t be opened.”
I could see it in my project list, but when I wanted to open it, it gave back a 404 error instead of opening like in the browser version of Figma. I than deleted the whole macOS app, redownloaded it and now the project seems to open just fine. Didn’t have any issues ever since.