Same issue here as well both in the desktop app, firefox, and edge

I'm experiencing the same issue. A couple of games on my wishlist that I cannot remove and a game I would like to add that I cannot add. Just getting 401 errors in the Network tab. I have tried both Firefox and Edge on Linux.

Open a support ticket. 401 Unauthorized is a server error.

You can try using a web browser to add and remove items from your wishlist manually. I don't expect it to work, but it only takes a moment to try. As an example, I'm using Pinball World below. You can find a more detailed explanation in remove invisible items from wishlist.

https://www.gog.com/user/wishlist/add/1207664403
https://www.gog.com/user/wishlist/remove/1207664403