503 Errors in JS/PHP

Hi all. I'm having some odd problems with some PHP/Javascript I wrote. The page http://fategen.com/mof/l1/login.php is supposed to get current login information in JSON format from http://fategen.com/mof/l2/login.php - but everytime I run the script, Firebug show l2/login.php returning a 503 HTTP error. If I copy and paste the URL being requested into my browser, it seems to work ok, though. A Google search suggested replacing the relative URLs in XHR.open() calls with absolute URLs, but this didn't change anything as far as I can tell. Could someone point me in the right direction on this problem?

Hi again,

I found out that this was an issue with mod_security in the log files:

[Tue May 22 14:05:16 2007] [error] [client 4.*.*.*] mod_security: Access denied with code 503. Pattern match "^$" at HEADER("Content-Length") [severity "EMERGENCY"] [hostname "fategen.com"] [uri "/mof/l2/login.php?nonce=2c0l86acfhl3r4wcck4k4gkgk&time=1179867915"]

Anyone have any idea why mod_security is choking like this?