At first, you can engage with Ra'khajin , then you can no longer target him.
Have tried leaving the area, /reloadui, logging out, quitting ... nothing changes.
Has this happened to anyone else?
Update: It seems if you simply wait (and wait and wait), the health bar comes back on the guy. Maybe something to do with the dragon overhead. Anyway, he got interactive again after something like 5 minutes and I got him.