So I just reallocated my points with the Soul Vessel. Now I can't seem to summon people when I'm human, or place my summon sign when I'm undead. Is this a known trait? Or are the servers just acting wonky right now? Or is something else entirely causing me to not join in on the multiplayer?

Never mind, I found out - I chose the wrong covenant to join :(

Sometimes, while in the process of being summoned, I get an error-message telling me I was "unable to join the multiplayer session". Afterwards cooperative play appears to be entirely disabled until I quit out and log back into the game. So ... try that, I'd say.

I get that all the time and it really sucks a dick.