Hello @paquat , I hope everything is ok and you have been able to solve the problem, if not I leave a recommendation for the first problem that the players are not appearing, the second seems more a temporary problem or a problem due to a spawning bug.
in the outliner you will find a configuration called island settings , which you can modify the maximum amount of players allowed in the game.
Also you must have a Player Spawner for each player , otherwise they will fall from the sky and will have to descend with a glider, if in your island there are 20 players maximum , it means that you must have 20 Player Spanwer.