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Most players have been complaining about this matter, maybe there is a fix with this dungeon? even players with high quality internet connection could still get randomly disconnected for no reason at all, and there are FPS spikes sometimes inside the dungeon, we don't know why that happens but atleast we could finish it and not get disconnected 
I've thought of a solution if you cannot fix this matter:
Let players reconnect in their party on the Nightmare dungeon when they get disconnected, Reward all the points they should have received by killing the npcs in the end of the stage after killing the boss, give 4 hidden chests to choose from the same as clearing a dungeon run, reward something like a chance to get random high grade talismans on golden chests, then just reward players with nightmare points of 300 on each silver chests chosen, this way players could play NM dungeon with ease, knowing that their count limit "SHOULD" not be decreased, and entering the dungeon would not waste 1 run limit, same on Nests, without finishing the dungeon, it will not consume 1 run.
I've thought of a solution if you cannot fix this matter:
Let players reconnect in their party on the Nightmare dungeon when they get disconnected, Reward all the points they should have received by killing the npcs in the end of the stage after killing the boss, give 4 hidden chests to choose from the same as clearing a dungeon run, reward something like a chance to get random high grade talismans on golden chests, then just reward players with nightmare points of 300 on each silver chests chosen, this way players could play NM dungeon with ease, knowing that their count limit "SHOULD" not be decreased, and entering the dungeon would not waste 1 run limit, same on Nests, without finishing the dungeon, it will not consume 1 run.