Chat problems, and lost connection

Hi! I bought Minecraft on another computer and then I got a new computer and logged in to my user and downloaded Mincraft for free on my new computer. When I'm on a server and writes in the chat function, I lose the lineup and are sent out back in the menu where you can choose "Multiplayer" or "singleplayer". When I go back joining the server again, I can not join. Could it be because I've downloaded Mincraft, free with the help of my old user? (Can I fix the problem if I buy mincraft again?)
I really hope you can help me fix my problem