... I've fixed it
It took some time for me to find out, but I did it
For those who even care; what I did is, I've reïnstalled my PHP Module and this solved my problem with the header always returning 500 Internal Server Error. With this header already set it was impossible to send 401 Authorization Required, so the browser didn't know authorization was required and didn't popup a "enter name and password" alert.
Mzzl, Chris