Error after submitting registration form

WP 3.5.1/ BP 1.7.2 / subdomain install / no core modifications / some hooks in functions.php

A number of our users (on Windows/IE) have reported an error after completing and submitting a registration on our site. <http://connect.projectlogin.net/&gt;

Some received a Gateway timeout, so we increased timeouts in our htaccess.

SetEnvIf Request_URI “index” DynamicTimeout=300
SetEnvIf Request_URI “wp-login” DynamicTimeout=300
SetEnvIf Request_URI “register” DynamicTimeout=300

Others are getting a 404 error. (We do not see these issues on any other browsers or platforms we’ve tested.)

I disabled all registration-related plugins, including Invite Anyone, but still have the problem.

I see some parallel discussions about forms triggering the IE 9 Popup Blocker.

<http://wordpress.org/support/topic/wpcf7_validate_email-triggering-ie-popup-blocker&gt;

However, I don’t know how to test where the problem might be coming from: WP core, BP, theme (Responsive), plugins.

If you have heard of similar problems with the BP register/profile fields screen elsewhere, could you post links?

Follow-up. One of our users just confirmed this problem on Mac Firefox and Safari:

After putting in my registration information and hitting the submit button it takes a long time to process and the result I get is the screen shot below. [404 Fancy Meeting You Here… back on the /register/ URL]