I tried Cogitation on BE 1.6.1 without any errors.
In your web.config file, there is a custom errors tag. Change it so it looks like:
After making this change, try viewing your blog with the Cogitation theme. If you still see a white page, then modify your global.asax file (in the root directory), and replace the Application_Error routine with the one I
posted here back on Nov 9th.
This should hopefully give you an actual error message, rather than just a plain white page.