I have a kind of a workaround for the problem talcabofrio was having: after exiting Soul Cairn and summoning Dexion with console commands I forced to start the quest Unseen Visions with console command "setstage DLC1VQ06 10". This will skip the dialogue with Dexion, so you will have to manually put the third Elder Scroll into your inventory. From there I was able to continue with the quest. I did not encounter a problem with Serana however, so I am not sure if this solution will work with that exact problem. Also I did this on the vampire side, so this workaround should work with both sides. : )
