Basically the door stays locked even though all the objectives have been completed.
The problem seems to be caused by there being a pathway that lets you get around the first door into the second part of the map before you’re supposed to. This some how glitches out the second door and prevents it from opening probably from completing the second set of objectives before the first.
I also have this problem. The grinning friends you need to kill don’t spawn at all or I killed them before the objective appeared.
I can confirm this, if you kill the mobs before the objective screen updates you cannot progress.
Same issue… Progress stopped at “Slay the Grinning Fiends: 1” The dungeon is clear and nothing remains. An the door is locked.
Same for me. The 'Grinning Fiends aren’t spawning or already dead. Can’t progress. 
Same here today. Still no fix.
Same here. Still no fix…?!
I killed one, second one seem not to spawn.
Same thing, still no fix
Same and i still need to kill 2…
wow, originally posted on June 5th, and this hasn’t been fixed yet. And I just watched a video with devs saying how after they create a dungeon they ask all the other devs to play it and then they brainstorm on how to improve it and/or fix it, and something as basic and fundamental as this is still in the game.
What’s happening is you can access the area beyond the locked door without knowing the locked door is there depending on the path you take. You end up killing the 2 mobs you need to open the second door while never seeing the ones you need to open the first door.
So you go back and kill the first two unlocking the door you already went past, and now there are no mobs in the dungeon, and cannot unlock the second door to progress!