for some reason I am unable to open port 443 with YaST. I am able to add the port number to the list of open ports, but it simply disappears after I navigate OK > Next > Finish. In the past I have been able to open other ports on this machine (20 for example).

This is rather peculiar because I am able to open any port (including 443) the exact same way just fine on other machines running the same system (SLES 12). The same happens regardless of whether I use the command line or navigate through the GUI.

This particular machine has Apache installed on it, which I suspect must be causing this somehow. What I'm trying to do is configure Apache to use SSL and opening this port is logically documented as a prerequisite.