I had the same problem

obywan <[email protected]> 13 Sep 2003 11:13:33 -0000
Newsgroups gmane.comp.web.oscommerce.installation
Message-ID <1512e7a9993ce8e0d3b175854a6e6567@osCommerce-Forums>
This message was sent from: Installation and Configuration
http://forums.oscommerce.com/viewtopic.php?p=228840#228840
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I had the same problem. Its an inherint bug for the latest version used on later versions of PHP. It can be fixed by following the below solution. 

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[email protected] @ 07/11/2003 07:30:07
This is a bug in PHP that shows up under certain circumstances.

Details can be found here:

http://bugs.php.net/bug.php?id=19553
http://bugs.php.net/bug.php?id=17236

Here are possible solutions/workarounds:

- Change the .htaccess file in the catalog directory. Remove the comment in front of the line reading
# php_value session.use_trans_sid 0

- Update PHP to 4.3.x - the problem is solved here.

- Force cookie usage

HTH
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I was able to fix it by changing the .htaccess file and forcing cookie usage. The updating of PHP is beyond my control, althought i did e-mail my web host provider to update their PHP... Hope this helps