Sorry if this has been mentioned previously, but I keep getting a "404 Not Found" when I click the link to someone's profile from my inbox.

Same here. It seems the link in the sMails section might be one that's no longer used? It's directing to "http://subeta.net/userinfo.php?user=" followed by userID.

Thank you for reporting this! I added it to this thread: * 404 Not Found nginx/1.4.4 Shops/Cults/sMail

And included it in my report to the programmers. :)