RE: our website requires a login to view, so do I really need the ROT13 extension?
If it requires a login to view, then no it is not needed.
RE: I deactivated the ROT13 extension. Now the emails display.
Another possible cause is that you might be using a caching plugin which has added a defer script loading option. The ROT13 script can not be deferred. So if you have a caching plugin and have enabled its defer script loading option, make sure you exclude the ROT13 script.
Hope this helps, let me know.