The join was aimed at a world opened to LAN, and never landed
Applies to the game client.
What it looks like in your log
Connecting to 192.168.1.20, 54321
What causes it
Minecraft picks a random high port every time a singleplayer world is opened to LAN, and this connection attempt used one. It never became a world join, which points at reachability rather than mods: Open to LAN reaches machines on the same network and nothing else.
How to fix it
Settle the network question before touching a single mod: are both players on the same router, or in different buildings? Same network, and it is the host’s firewall, so allow Java through it and retry. Different networks, and LAN was never going to arrive: the world needs a real host or a tunnel between the two machines. If this port belongs to a hosted server rather than somebody’s singleplayer world, ignore this line, because the two look identical from the client side.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log