Getting a Page not found error when trying to change settings

Hi!
I'm getting a "Page not found" error when I try to change settings like the number of emails to send on each cron run.
I've disabled clean URLs but I'm still getting the same error.
There was a strange side effect: my IP was blocked in the server's firewall after a couple of times trying to submit that form.

I really need to change that setting really fast. Is there any way to do it (like messing the DB value) while we sort this issue out?

Thanks!
Ricardo