Hab's herausgefunden :-)
Auf php.net/header findet man folgenden User-Kommentar:
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18-Jul-2002 07:31
This may be obvious to some, but to send HTTP status code headers to IIS
(i.e. unauthorized 401) use the "Status:" header and not
"HTTP/":
header('Status: 401.5 Unauthorized');
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Grüsse, Arash