Is the Allow access to the portal through HTTPS only check box, unchecked? You could follow these steps.
1. Sign in to the portal website as an administrator of your organization. Click Organization and click the Settings tab.
2. Click Security on the left side of the page.
3. Portal for ArcGIS enforces HTTPS communication by default. To allow both HTTP and HTTPS communication in your organization, uncheck the Allow access to the portal through HTTPS only check box. If you have previously unchecked this box, rechecking it will once again enforce HTTPS only communication for your portal.
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