So - i tested it profusely.

I went in after verification without mods on Singleplayer. It worked - fairly a bit too well, since i was overrun by spiders on the first try (about 50). On the second there weren´t as many. (This at least feels suspicious) The first miniboss still was teleporting somewhat strangely around until he finally showed up but - he showed up. (As I said, second try after I revived in the dungeon - doesn´t feel quite right)

I tried again with mods (Singleplayer) - it bugged out again. So yes, maybe a mod might be the problem here. But - and I still don´t get it - it worked before the New Chapter just fine even with two mods more on our server. So if there´s a problem with a mod, finding out which one it is after the update of the devkit is quite timeconsuming. And I don´t have the time at hand to test which one it is and- not to mention - i am not our private servers administrator to “F around and find out” which one it could be.

In MP it was the same - whatever mods not working there, the fight doesn´t work properly after the devkit-update/ new chapter if you play with any of the mods above.

I will not go onto an official server to test that out, maybe another player is willing to test the fight on one of your official servers, if they can.

At least now I know that one or a few of our mods bug the fight out for unknown reasons.