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Though the reasons can be plentiful when it gets down to the detail, the high level answer is that your web server is likely routing to something different over port 80 than it is on port 443. If this were Apache and a static HTML website, it could be as little as having two virtual hosts for the two ports, each one pointing to a different DocumentRoot. This is going to vary by your setup though. Can you tell us more about the application stack you are running?
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