"curl" makes form submission result in 404

[eluser]@li[/eluser]
The first few errors about uri_string() look related to the show_404 change, but this error:

[Sun Oct 12 20:16:48 2008] [error] [client 74.6.18.229] Directory index forbidden by rule: /home/mickm/public_html/tamamari/

looks interesting. Do you see a lot of these errors? If so, it could be possible that accessing index.php is disabled by your apache configurations and that is why you get a lot of these errors. Its also possible that your script throws a lot of errors, and instead of showing them apache throws an internal 500 server error (some servers do this).

Can you try one final thing. Restore the original 404 error then make one more test form submission to get the error again. Then check if there are any new entries in the logs and post here. If not, we can contact your web host about this.