Most of the time when I deploying a sandbox platform, I don’t bother with configuring HTTPS. I had this problem for the fist tine not long ago, because as you know, with p8 5.2, the FEM became almost useless. But I needed it anyway and had this problem:
http://hostname:9080/wsi/FNCEWS40MTOM' HTTPS is required
After some research, it actually comes from WCF which requires a secured connection to work. You actually just have to install WSE, which is the ancestor of WCF and does have this requirement. This is for test platform only obviously because of the security issue, but for sandbox without security issue, it can be nice to not have to set up SSL.
You can download WSE here, install it and just try to connect again with FEM, no need to reboot or anything