Create a new world on 18w20c with server.jar 18w20c, start playing in solo and when a second player join the game the server crash with the message "Connection Lost Timed out" on the client and this a the server console :

[17:22:58] [Server thread/INFO]: Thornin[/192.168.1.21:51021] logged in with entity id 333 at (252.21291172132706, 55.0, -132.69999998807907)
[17:22:58] [Server thread/INFO]: Thornin joined the game
[17:29:00] [Server thread/INFO]: [Thornin: Set the time to 1000]
[17:29:41] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3965ms behind, skipping 79 tick(s)
[17:29:55] User Authenticator #2/INFO: UUID of player Charlotte71 is c17c9e08-9aee-4b6d-9f7a-800b54bb42c8
[17:29:55] [Server thread/INFO]: [STDOUT]: eg{x=240, y=64, z=-132}
[17:29:55] [Server thread/INFO]: Charlotte71[/192.168.1.23:50489] logged in with entity id 79468 at (246.60083513027655, 64.0, -144.1360688403744)
[17:29:56] [Server thread/INFO]: Charlotte71 joined the game
[17:31:02] [Server Watchdog/FATAL]: A single server tick took 60.00 seconds (should be max 0.05)
[17:31:02] [Server Watchdog/FATAL]: Considering it to be crashed, server will forcibly shutdown.
[17:31:03] [Server Watchdog/ERROR]: This crash report has been saved to: D:\Minecraft\Serv_1.13snapMesa18w20c\.\crash-reports\crash-2018-05-21_17.31.03-server.txt
[17:31:03] [Server Shutdown Thread/INFO]: Stopping server
[17:31:03] [Server Shutdown Thread/INFO]: Saving players

I don't know what you need ... What can i do for help ? i've also joined the crash log