ok, so you need to play/wait in a map, in order for the HNM bosses to respawn. if your map doesnt have the boss in solo mode, then you wont have it on multi mode

The thing is it had spawned in single mode and in the map selection in multi mode it said it was there, but when i entered the map it suddenly wasn't there anymore, no warning massage, no HNM, just the normal mobs. I went back to single mode and there it was, i could run in the red circle and start the event. So back again to multi mode and map selection again said "yup, your HNM is there happy farming" but when i entered the map, AGAIN nothing. After that i gave up. When i started the game the next day everything worked just fine, so i guess it was just some kind of bug.

yes, then that's a bug, dunno what's wrong there, probably you will need to kill it once first, but usually not like that in order to make it spawn in multiplayer/ Union