Attempted to change theme - Invalid form token. Please reload this form and submit again.

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Hello
I am attempting to build a custom theme for Concrete5. I have built several in the past, however the most recent site, when I go to themes =>change theme, it asks if I want to apply the new theme to all of the pages. When I click yes, it says "The page is unavailable or does not exist. Any thoughts?? Could really use some help!