private server crashes when a player logs in

First off I am sorry if this bug is already on here.
I searcheed the site but couldn't find this problem.

I am running a private server with a whitelist.
it was running fine on 1.9.1 when a player that had been playing hours before enters the server and it shuts down. I updated to 1.9.2 and everything was fine for a few hours and then the same thing happened.

I can not reproduce the bug on purpose but I know the following:

• the player was not using mods
• the server is not using mods
• the player could enter the server without problem hours before
• he entered the server and it crashed instantly.

I attached the crash report.