The Copper Citadel can't be completed

Hello,
Tried 2 Copper Citadels so far and both towards the end of the boss fight just got an unexpected disconnect and I'm thrown out at Login menu and of course the map is considered as LOST on the atlas, when I was winning. Extremely frustrating. Thought I got unlucky first time, but when second crashed at almost the same point (Boss has around 20% health left) it feels like a bug.

To be noted that other Citadel's were able to be completed with no problems or disconnects.