I have a fresh Portal 10.5.1 installation on Linux CentOS 7.3. Calling the portal on http://machine.domain:7080/arcgis/home the page appears but immediately switches to https://machine.domain:7443/arcgis/home/createadmin.html and the page shows "Initial Administrator Account Already Set Up". This happens for any page, eg. http://machine.domain:7080/arcgis/home/webmap/viewer.html The security "HTTPS only" option is false. Everything works ok if accessing only through 7443. I also tried installing the webadaptor, but when I try to access http://machine.domain/wa/home it switches again.
Solved. Installing real certificates solved the problem, while with self-signed ones it was redirecting.
Although, since I had to rename the host doman e reinstall the Portal I am not sure that by simply installing real certificates solves the problem.