I thought it was a server side issue, but the problem has persisted for me for over a day. Over the 60 hours of my first playthrough I never had a server issue.

But now I can neither summon, be summoned, invade, or duel.

So I'm thinking its one of two things: I joined the covenant of champions just to get the trophy then abandoned it and joined a new one. Maybe the restriction on summoning persists (bug?) even if you leave this covenant?

Or, it's because I've beaten the final boss. Any ideas? Kinda bummed I can't continue to duel...

Do messages and bloodstains also no longer appear? I've noticed for me that sometimes during a failed invasion with either a red or blue eye orb, the game will inadvertently shut off the online services. Though from my experience simply quitting and loading back in fixes the problem.

Messages and blood stains still appear, and I've tried restarting several times.