Just saw this post while I was checking all the subforums. I have the same issue. It's not browser specific, nor OS specific, though. It happens in Windows, Android, Linux, and Mac using Firefox, Chrome, Opera, Internet Explorer and others. Clearing the cache and using incognito / private modes doesn't change it.
Every once in a blue moon, the unread posts link works, but it mostly doesn't. I just stick to the main photo/image subs (Phrag, Paph, Other), and then from time to time click into the other subs to see what's new (since those are far less active). Alternatively, I'll open ST in an incognito window, and just won't log in, so that there's a link to Today's Posts. However, I suspect that doesn't work correctly as well.