Strange 404 in wp-admin/pages.php

WordPress version is 2.9.1 without any plugins, updated from WP 2.6.2, the problem appears in both versions (in 2.6.2 it eventually crashed the server. On 2.9.1 I get a blank page).

I cannot save some pages. When I hit “update” Firebug Network Console gives a 404 error for page.php. The file is obviously there or I wouldn’t be able to open the edit screen in the first place.

This happens only on a few pages and seems in some way connected to the contents of the page, yet I can see not pattern. One page will not save unless I delete the string “(2009)” from the content. One page does not save unless I delete the word “park” from the content. And so on, I think you can see why I’m puzzled.

The site runs flawlessly on other servers (with much more restrictive values for PHP memory limits, execution times etc), yet I do not want to move servers without knowing what the problem is.

The most obvious difference between the current live server and my test servers is that the live server has the suhosin security patch for PHP installed (http://www.hardened-php.net/suhosin/). I do not know if that matters, though. I contacted the web hosts tech support but to no avail.

Can anybody help ?

Thank you for your time,

eike