[quote=gigamo]Alright, so I figured out what the problem was in my situation.
Basically, one of my friends stopped playing around the time we reached Driftwood, and his character has therefore just been tagging along. That character thus did not complete all the rituals inside Meistr's basement, and that is what the Arbiter was talking about. Once I read those two tablets (one for Source Vampirism, one for Spirit Vision) while controlling that character, it also learned the skills, and the Arbiter finally let us through.
TL;DR: Make sure all your characters have both Source Vampirism and Spirit Vision skills unlocked. [/quote]
Yep, that fixed it for me. Thanks!