It’s pretty much impossible to join a multiplayer game for the Crucible because you have no way of telling from the server browser if the Crucible is active and you get kicked when trying to join an active game. You have to get lucky to join the game in those 3-4 seconds it would take the players to start the next ten waves.
I suggest two possible fixes:
-when you try to join an active game, you are placed in some sort waiting queue where you will automatically join the game as soon as the waves stop (best solution)
-indicate in the server browser whether the Crucible is active and/or allow to filter out active games that you cant join anyway (easier to implement I imagine, but annoying because you would have to spam refresh to find joinable games).