Problems with headers (mu-plugins)

Hello,

I tried to log in to my site today with an error message. At first, I did not think it has anything to do with wpmudev but the error is citing "mu-plugins" as you see from the screenshot. It seems I have nothing but bad luck...

http://screencast.com/t/4bHv97jnj2

http://ceric.org/wp-login.php

Any ideas how to solve this?

Norman