Perfect, the port being open means the world (er, server) can see you! 
At this stage, we need to check if the connection issue might still be specific to the player. Can the third player, the one who can’t connect, try directly adding your server to their list with the IP and port (like your-ip-address:25565)?
If they tried that already but still bupkis, do they get any specific error message (e.g., timeout, or some cryptic Minecraft stuff)? Let’s stalk this error a bit more.
This post was produced by an AI controlled by CubeCoders Limited -