OK, I played a bit, and decided to advance the Ryberg speak with Dead Quest. After accepting Ryberg's call for help, I eventually found a scroll of "Speak with Dead". Ryberg was waiting in Waukeen's Promenade, and I gave Jan the scroll and had him speak with Ryberg. The first time I tried it I forgot about the cowled wizards, and it triggered a teleport hunting party. Oops. I reloaded, and this time I selected the option to discuss this in the Graveyard. We all transition to a tomb indoor area, Jan reads the scroll and the "Spirit" gates in. We select the option to speak with Ryberg's dead relative Uncle Donny. Spirit performs a casting/gate animation, But then nothing more occurs. Donny doesn't appear, and if you talk to the spirit again the conversation loops back to previous. Ryberg says nothing. The spirit is still just standing there. Something misfired here. I had the same problem on my last 2.5 version run, and now I'm doing a 2.6 run.
I try the console, and create Uncle Donny, and the dialogue works for a few lines. He gives his clue and disappears. But only previous dialogue from Ryberg will display, and the proper new text isn't being triggered. And of course the Spirit is still there (I believe he is supposed to gate away earlier).
Tiring of this, and not sure if it is worth messing with, I go back to the save game and try a different dialogue option, and tell him I don't think it is a good idea to speak with his dead relative, and I give him the scroll. I get 1,000 XP, and the quest is over (although the journal entry has no update, which may be another glitch here as well).
This is just an FYI, in case someone else has the same or similar issue. I'm running SCS and SR, and a load of other mods, so it may be that, or not. There is one thing that might be the cause, and that my PC is a Shadow Warrior (a FM), so he can cast Shadow Magic, but not Arcane Magic. If the CtB module is looking for the protagonist to be a mage, then this could cause an error perhaps? Just thinking aloud here.