What settings can block a login submission

What settings would need to be changed that would keep me from being able to log in/ fill out an order form etc. for everywhere but in these forums that I go to.

At first this only applied to when I use IE but now Opera is starting to do the same thing.

Note that I can access the log in pages and enter my log in details but when I click submit I get a page not found page.

I have checked the IE settings butfind nothing that should be causeing this.

Thanks for any ideas.