To julio, regarding the (deleted) message "REMOTE_USER"
I found out by myself a couple of minutes ago:
just because I wrote "\n" (or "\r\n") instead of "\n\n" after the "Content-type: ", I got the server error! :-(((((
It works now :-)
As both you and ozo had the correct idea, I thought I'll give each one of your 4*30 points (splitting the 60), so please do answer this question.