Connecting to a friend's server in multiplayer mode make appear in the ground,

I was playing with a friend on a survival server I've created a few days ago but since version 1.6.1 my friend get problems when he connects to my server. His character appears in the sky and then he falls in the ground and stay caught. He can still break the ground but can't move. Else, from my screen, he looks frozen and doesn't move. Only his arms are moving when he clicks but there is a delay for about 4-5 minutes until I see what he did.

We tried creating a new server and the issue still happens even if we return to version 1.5.2. Finally he did a new server and I tried to connect to it but the same issue occurs on 1.5.2 and 1.6.1

Also, when he's caught, the chat is invisible. See attachments.