After upgrading to the new 1.12.1 version of spigot, players disconnect with the error message:

Internal Exception: java.io.IOException: The external host has ended the connection

However, when I stop the server, place the 1.12 version back, and start up the server again, this failure does not occur.

Furthermore, the player seems to connect to the server and then disconnects from the client side, without any errors being thrown on the server side (see logs):

 

[17:35:10 INFO]: UUID of player NickvdMeij is 9e4e91a7-8226-4de1-9527-2b43fde45ab3
[17:35:10 INFO]: NickvdMeij[/10.42.16.155:42834] logged in with entity id 1760 at ([world]660.4625740142916, 84.68088355940982, -385.63593708292393)
[17:35:32 INFO]: NickvdMeij lost connection: Disconnected
[17:35:32 INFO]: NickvdMeij left the game