are you logged in when you try to access that page? seems like a dumb question, but i know on my site (from rather laughable experience) that a currently registered user who is logged in can’t access it. anyone not logged in can…just double-checking!
chances are you have something in your theme’s functions.php file causing the problem. what i did was remove an item, save the file, test the site. do this for each entry in the file – it could be something like a spelling error, etc.
I’m having the same issue. my theme is updated to 1.5, i deactivated it and activated the default theme, and i deactivated everything save buddypress and akismet. also just reuploaded 1.5 fresh. no activity feed…