Have already had 2 instances of this problem when doing TA Exp with groups, I agree it is relatively easy to solve (i.e. instance starter leaves and rejoins) but this is rather sloppy and hope it will be fixed sometime soon. The bug in question activates whenever the 1st dialog option with Morrigu (asking her about the nightmare court) is chosen before the choice to open the Exp path.
http://youtu.be/n7ZZ53X64ag