Strange problem with block-random.php

I have a strange problem with block-random.php in combination with a
SSI (shtml) page. When I use block-random.php in a shtml page, I get scrambled signs in stead of a (random) picture.

The text is like: Pj@ zmJ. uaveJ!)mh/= H3Q?Qb*c yj=E&= etc.

This problem only exists on a Windows 2000 client with MS IE and NO proxy server. If I use a proxy server, there is no problem. A Windows XP client with the same version of de MS IE and no proxy server gives no problem and shows the picture perfect.

If I don't use a shtml page in the faulty situation and use the block-random.php directly in the browser, there is no problem.

De site runs on a Windows platform with Apache. I use the code: <!--#include virtual="cgi-bin/gallery/block-random.php"--> to get the block-random.php from a shtml page.
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Gallery version: gallery-1.4.4-pl2
Apache version: Apache/1.3.29 (Win32)
PHP version PHP/4.3.3
Graphics Toolkit: netpbm
Operating system: Windows
Web browser/version (if applicable): IE 6.0.2800.1106

Anyone?