Richtextbox problem

Question

Hello,
The last few days I've been busy trying to write a chat/chat server and so far it's been great, but now, that should be the easiest part, it goes wrong.
The last hour I have been having problems trying to add text to a richtextbox.
The string which I'm trying to add shows up with a messagebox, so it's really this line; this.richTextBox2.text += data;
I have also tried;
this.richTextBox2.AppendText(data);
this.richTextBox2.text = data;
And any other form that I know of.
Can anyone please help me?
The whole function can be found here: http://pastebin.com/m33337a4c