We started hosting a vanilla 1.17 server for me and a few friends and the fist day everything went well but when trying to connect the second day, two of my friends kept falling through the world on their client, but the rest of us could see them standing still and not moving. This led us to believe there was a connection problem, but we could not resolve it. We tried restarting their router and the minecraft server (several times), disabling antivirus and deactivating their vpn, but nothing helped. They even reinstalled minecraft. We're out of ideas and so I thought why not ask here?