Can't join LAN world

My friend cannot join my world and I cannot join theirs. We are both on the same network/Wi-Fi and use the same version of the game, 1.20.1. We tried using the Open to LAN button and that still didn't work.

We can see the game on the Multiplayer screen but when we try joining it, it is stuck on the connecting screen, and eventually stops with an error.

I usually use Lunar Client and my friend usually uses OptiFine in the minecraft launcher. I tried using the same optifine version and exact same launch parameters and that still didn't work. (Edit: Unmodded vanilla "Latest release" java doesn't work.)

It worked once a few days ago and now it doesn't work at all. Please help! Thanks 🙂

This is for java! Bedrock works completely fine for us (LAN)d{*}{*}