Client crash when join my server and after the server don't work (Red Box)

Like I say on the title… automatically someone crash on the server i have a lot of error on the console like this : System.IO.IOException: Read failure —> System.Net.Sockets.SocketException: Connection reset by peer

I run the server on a VPS (Ubuntu) and this error is truly random, sometimes it’s when someone joins and time out or sometimes that’s when I disconnect

https://gyazo.com/59dace3519334cd537759743da8704fe

*Sorry for my bad english

Update

I join my server,
I disconnect
I try to reconnect (Don’t Work)
I close Fivereborn

and few second after I close FiveReborn, the server ‘Crash’ (Make error and after when i join i have a red box)

**Edit

The problem is my Ubuntu, I try on my Windows and no problem… I wanna to try to change the version of Ubuntu