I have a 2023 M3 Macbook. I have used it successfully all year to play Geforce Now games.
3 days ago I tried to open Fortnite and it says "to play a game on GeForce Now you need to increase the UDP buffer size on your mac." It links to a debug page with some steps to do on terminal to make sure your buffer size is high enough (8MB) and to set it there if it isn't. My buffer is set at 8MB so that isn't the issue.
It also happens when I try to open other games and it doesn't matter if I decrease the launch settings from competitive to data saver etc. Also I have "ultimate" membership if that matters.
No settings have changed except a Geforce Now update from when it worked. Everything else is exactly the same. Same network. Same Location. Same computer etc.