I'm also having this issue with friend requests. It doesn't matter if I'm the one who sends them to others or if others send them to me, it won't let the request be accepted by either party. It doesn't matter if it's on the website or in-game. I've resorted to adding people through Steam but I've met some people on console I would like to add. My friends list isn't full either. I haven't been able to find a solution anywhere, I hope Bungie is able to fix this issue.