Cannot add web parts to pages located in _layouts folder

Question

I am developing a website in SharePoint 2007 where all the .aspx pages are located in a subdirectory within the _layouts directory in the 12 hive.

I have designed a custom masterpage with corporate branding and I am using this masterpage in the .aspx pages. I have found that I cannot add web parts(both in-built and custom) to these .aspx pages. I get the error: "Unable to add the selected web part(s). The security validation for this page is invalid."

Is it that we cannot add web parts to .aspx pages that reside within the _layouts directory of the SharePoint's 12 hive?

Please help.Thanks in advance.