Your XSN is just a .CAB file that is renamed to .XSN.
Change the extension to .CAB, extract all the files into a folder, edit manfiest.xsf with notepad, and remove the <xsf:header> section, then save.
Now right-click manifest.xsf and choose Design. You should be able to get the form open in the designer now. Save it back as an XSN.
Maybe your header had some unsupported content? Try creating a simple header with just some text "abc" and see if that works... then go back and try to re-add your full header.