WPMU can not signup on IE with 404 error

Yes, I learn to install WordPress MU on the 1and1 server.

I finished the installation very smoothly. Only one issue found by my friend George yesterday. He told me that the wp-signup.php does not work.
Actually, I have no problem on my computer with Firefox.

I just have an idea, it may be the IE. Ok, I click the ugly IE icon to load http://blogs.peaceever.com/. The wp-sign-up.php return the 404 error.

It is so strange.

I doing some search by Google "wp-signup.php ie 404".

I got the solution.
Open .htaccess in your WPMU folder.
Add the following line at the top:

AddType x-mapp-php5 .php

It works.

I don't know why it happened and what problem it come from. I just fix it.