I know this is a couple weeks old but this just happened to me in the exact same way (Koldrak, reached the dragon, lagged a few seconds then the disconnection message) it super freaked me out and i immediately looked it up and found this thread, i'm sort of relieved now that i'm not the only one who had this happen.
I guess the only thing i would like to ask to you Nyxsm is if this has ever happend to you again, i'm now sort of scared of trying to join a Koldrak run. If you did contact support, did they had anything to say about it?