403 - The website declined to show this webpage

Hi, 
I've been for 2 day, looking (in the worng places....) and found no solution or something that would help me to solve this problem.
This is the scenario: IIS 6 + ASP.NET 2.0 WebSite, Default Web Site with Some Virtual Directories...
With one of those Virtual directories, I have an issue. I log the user (web.config with Forms Authentication, IIS with Anonimous Access, directory browsin), go to a menu page, click on a button to redirect to another page and there I have a form with a button.
When I click the button I get the  
The website declined to show this webpage HTTP 403 <div cla ...