I have installed DNN 4.0.2 and have completed considerable development work within the development environment.  I am now ready to publish a version of the site to my staging server.  This is all fine.  The site is precompiled and seems to function fine until I go to edit something.  I log in as host and choose to edit a text/html control.  I get the error Value cannot be null. Parameter name: format.  I am figuring since I have not seen this error on here, that I must be doing something wrong.  Before you ask - I have granted modify permissions for the Network User.  The only thing that might be different is it is running in its own application pool.  I have tried and tried but cannot figure this one out.  Please help.

I'm seeing this post http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/107/threadid/14873/scope/posts/Default.aspx

I will see if it solves the problem

It worked for me.