No that's not the problem. I have more than enough stamina. Even with a full bar and with the daily stamina bonus I don't get further.

I killed Molten Golem and quited the quest. When resuming the quests later, my order was to kill all enemies. I came back in a previous level of the quest, killed the only monster that was alive in that level, went to the gate and came into Molten Golem level. There was everything killed already and the rest is what I told before.