Well, i had problems with this quests too.
The worst thing was, i lost the option to go to another map. The quests lead into new maps, thats what i dont like. If i miss any quest because it wasnt that obvious to find or i just dont want to quest and just grind to level up. There must be the option to get to the further maps without finding and complete those quests.
I took me a lot of time finding the problem, just to find out i missed the quest or had a bug in quest completion. The problematic step was where u have to defend the bridge before you can get to the mines. my groupmate must have took this quest but i was somewhere else i think. somehow i could get into the mines as long i was grouped with that guy but when i left the group the bridge was still blocked and i could get into the mines per porting system. it was weird because i could do some quests inside the mines but not the whole line to get the new maps unlocked.
Maybe this helps in any way.