Error - "Forbidden" message when updating quantity in shopping cart

A wholesale customer with a large quantity of items in their cart emailed me because they are getting an error when they try to adjust quantities. Hitting the update button sends them to index.php?main_page=shopping_cart&action=update_product, but the page is white and says:

"Forbidden
You don't have permission to access /shop/index.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."

I used my master password to login to their account and it is happening to me as well. I logged into a different wholesale customers account, added items to their cart, and was able to adjust quantities just fine. I was using 1.3.9 and thought maybe that was an issue, so I upgraded to the newest version. Everything else seems fine after upgrading, but I am still having this issue and only with that particular customer it seems. Is it possible to have too many items in the cart? They currently have 549, but the cart has never done that before even with higher quantities. Any idea what's going on?

What modules do you have installed in your now 1.54 cart ?

Database Backup, Dual Pricing, Master Password, Super Orders, Column Layout Grid. A lot of my research shows there might be an issue with mod_security so I've contacted my host. I'm not familiar with mod_security. It's so odd to me that it's only happening with this customer (so far). They have placed orders just fine for years. I've used Zen Cart for years and I can usually solve my problems, but this one is driving me nuts!

My problem has been solved so I thought I would mention it in case anyone has a similar problem. This ended up being a mod_security error. You need to talk to your web host in order to fix it.

57399:

My problem has been solved so I thought I would mention it in case anyone has a similar problem. This ended up being a mod_security error. You need to talk to your web host in order to fix it.

Thanks for the update :)