Minecraft Java Realm, Forced Connection Error by Remotehost

I have started the Test Trial for Minecraft Realm, 3 of my Friends can Join without any Problem, but ma 4th Friend cant Join on the Server he needs to Try to Join at least 20 Times until the Server lets him Stay on the Server, he is the only One getting this "Forced by RemoteHost" Error the whole Time.

I got this Server/Realm Error too but it was just on 1-2 Times now on over 10 Days, why ist that so? 3 Can Join without any Errors just him is Constant this Error Popping up