Concurrent Mod Exception

So I've been working on this program to make tournament matches for beach volley.

First little info: (I'm Belgian so some words are Dutch)
-Class Speler: has arraylist with information with which player already has been played (codesLijst)
-Class Inschrijvingen: makes players sign up with their level and name
-Class Wedstrijd: makes matches, checks with Inschrijvingen if the players already have played together. If so, gives message that they need to change partner. If not, makes them a team.

Now if I run the program with fresh players, none of them have had a partner, it works fine. If I make just one new team and the other already exists, it also works fine. It's when I make the two teams with existing records (to see whether they already played together), that the error comes.I know there's a lot of code duplicate, will clear out eventually, this is just a raw version.

CLASS SPELER

import java.util.ArrayList;
public class Speler {
private Wedstrijden wedstrijd;
private Inschrijvingen inschrijving;
private String naam, ploeg;
private int setPunten, winPunten;
private long time;
private String code;
private ArrayList<String> lijstCodes = new ArrayList<String>();
 
[code]....