"The server you were prviously on went down..." happens all the time

Hey,

I have a Bungeenetwork with 4 Servers: Lobby, Skywars, Bedwars and KitPvP.
Anytime a player dies on KitPvp - a round of Skywars starts or a round of Bedwars ends the players get kicked with following message:

The server you were previously on went down, you have been connected to a fallback server

But the server does not crash.

I do not remember something I could have changed that leads to this error.
No errors in console

Sorry for my bad English. ^^