"The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again." This error message in sharepoint 2007 you will get it because of different actions you may perform on the site level:
-If you are trying to check out a document in Document library.
-If you are trying to add some columns in a custom list in sharepoint.
-If you are trying to call update() method in the object mode.
-And other actions....
The reason behind this error is when you enable anonymous access to a site with other authentication method such as basic authentication.
The solution or workaround for this problem is to open the central admin, then :
- Select application management.
- Under SharePoint Web Application Management, select Web Application general settings.
- Select your web application first.
- Under web page security validation turn it off.
- Reset your IIS.
Hope this helps.