Once the reactor stabilizer arrays are destroyed, the exarch is supposed to flee to the next one. Instead, the exarch didn't move, and was stuck invincible. I've tried resetting and abandoning the mission, but we're forced to start in the same place as when we left off. So as far as I know, Hoth can't be completed. Any ideas?

Resetting the quest won't help, you need to reset the phase to start from the beginning. You can reset your phase in the portrait menu; just right-click your portrait (bottom left on the screen) and hit "Reset all active flashpoint phases". Edited January 9, 2016 by Jerba

Resetting the quest won't help, you need to reset the phase to start from the beginning. You can reset your phase in the portrait menu; just right-click your portrait (bottom left on the screen) and hit "Reset all active flashpoint phases".

 

This.

 

Had the same thing happen to me last week. Had to reset it, and phases. Worked fine the second time.