Log-in Issues

I'm a Zen newbe so I apologize if this is a stupid question...but here it goes:

I just finished getting my new Zen store (latest version) installed and setup. I use FireFox and it worked great. In my excitement I emailed everyone I knew with my URL. Most of them use Internet Explorer...

When using IE (but not FireFox for some reason) you get the following error when trying to log in:

Warning: strstr(): Empty delimiter. in /nfs/cust/.../web/includes/init_includes/init_customer_auth.php on line 15

I do not get any log in errors on the admin page log in even with IE.

line 15 of the file listed says this (if my count is right):

if (!strstr(EXCLUDE_ADMIN_IP_FOR_MAINTENANCE, $_SERVER['REMOTE_ADDR'])){
// if (EXCLUDE_ADMIN_IP_FOR_MAINTENANCE != $_SERVER['REMOTE_ADDR']){
if (DOWN_FOR_MAINTENANCE=='true' and $_GET['main_page'] != DOWN_FOR_MAINTENANCE_FILENAME) zen_redirect(zen_href_link(DOWN_FOR_MAINTENANCE_FILENAME));
}
/**

Any help would be greatly appreciated :smile: Thanks!

Just a couple more details if they help:

-The above errors occure in IE if you are not logged in, add an item to your shopping cart, and then hit the check out button.

-After hitting the check out button you are taken to a blank page with this error message.

I've been reading the forums like crazy and have elimenated these possibilities:
-I have a valid IP address for my maintainence override page (I think)
-There aren't any extra spaces at the begining or end of the files listed

Bumping in the hope that someone can help...