XMLHttpRequest 404 but response OK??

Hello,
what’s new with URL query string handling in WP3/3.0.1?
In 2.9.2, I was able to include other pages via ajax with javascript, mootools and a loader php page in my template, now it seems that custom URLs don’t work anymore.

The ajax inclusion of
http://www.domain.com/wp/wp-content/themes/mytheme/ajaxloader.php?pagename=my_page/my_subpage
is OK

but using custom query string variable names and values (usually later processed by the loader php)
http://www.domain.com/wp/wp-content/themes/mytheme/ajaxloader.php?custom_variable_name=my_page/my_subpage
http://www.domain.com/wp/wp-content/themes/mytheme/ajaxloader.php?pagename=dummy_value/my_page/my_subpage
it throws out 404 errors even if the actual content is correctly shown by Firebug in the response content and html tabs.
Also, opening the link in a new window will work as expected.

It sounds like a kind of cross-checking of default variables and corresponding existing values. How could I find a workaround?