There is an issue when you are grouped with at least one other person. When Zerith Var makes a portal, it disappears before either of us in a group can use it.
For instance travelling to Kenharthi's Roost. The portal appears for about half a second, then PooF! gone.
This has happened the 3 times we've picked him up. Every single time. Though one instance, one of us could travel and the other could travel to the grouped player, and it would advance the quest.
If you are solo, no problems. Also no problem if you are grouped but the other person is offline.
This is on Xbox EU.