Server Crashes When certain players join - java.lang.StackOverflowError

So i own a server that i made using the java server file that you can download off of the minecraft website. on this server there is world with about 15 people, i leave it running all day for two days then i turn it off to back up the world then turn it back on. i have been playing on the world smoothly for about a month and a half when suddenly the server just crashed. i tried loading it back up but it just crashed as soon as a certain player joined. this problem happened to two other players. is there a way to fix this without damaging the world?

 

I will attach any attachments necessary to fix. i attached the world file and i attached the crash log. please help!