Thank you for the post.
"Is the low RAM the likely problem?"
The symptoms described indicate a lack of system resources to successfully run WebDirect. 4 GB of RAM is the minimum recommend amount if not using Web Publishing (Database Server only).
The minimum amount of RAM for using FileMaker Server 13's Web Publishing (WebDirect/PHP/XML) is 8 GB.
Thank you TSFalcon. I upgraded to 8 GB, and WebDirect is now enabled and working. I still cannot connect from computers that are not on the same ethernet network as the server, but I believe this is due to my employer's firewall - hopefully they can open the necessary ports.