Collection was modified; enumeration operation may not compute

The following is a script for my enemies in a 2D game that looks at certain points. The majority of the time it works, although every one in a while (1/5ish) the error. I can't find any way to replicate the error consistently, sometimes it throws once every time the coroutine runs, sometimes it throws multiple times. 99% sure the error isn't due to the layermasks or tagging, 100% sure not do the allWayPoints list. It is a little complex but any help is welcome.

 InvalidOperationException: Collection was modified; enumeration operation may not execute

It says the error is at line 778 in the file List.cs.

 //Needs work (buggy and glitchy sometimes)
 IEnumerator lookAround(float time)
 {
     visiblePoints.Clear();
 
     //Raycasting and adding to visiblePoints list
     foreach (GameObject obj in allWayPoints)
     {
         RaycastHit2D hit = Physics2D.Raycast(transform.position, obj.transform.position - transform.position, Mathf.Infinity, layerMask);
 
         if (hit.collider.tag == "LookPoints")
         {
             float dirAngle = Vector3.Angle(transform.right, (hit.transform.position - transform.position).normalized);
             Vector3 cross = Vector3.Cross(transform.right, (hit.transform.position - transform.position).normalized);
 
             //Checks which direction of angle
             if (cross.z < 0)
                 dirAngle = -dirAngle + 360;
 
             Waypoints newPoint = new Waypoints(obj.transform.position - transform.position, dirAngle, hit.distance);
 
             bool checker = false;
 
             //Makes sure visible points aren't too similar of an angle
             foreach (Waypoints item in visiblePoints)
             {
                 if (Mathf.Abs(dirAngle - item.eulerZ) < 20)
                     checker = true;
             }
 
             if (!checker)
                 visiblePoints.Add(newPoint);
         }
     }
 
     //Sorting
     visiblePoints.Sort();
 
     //Reversing if necessary
     if (360 - visiblePoints[visiblePoints.Count - 1].eulerZ < visiblePoints[0].eulerZ)
         visiblePoints.Reverse();
 
     //Looking at each point
     while (timer < time)
     {
         //So it runs at least twice
         for(int i = 0; i < 2; i++)
         {
             foreach (Waypoints item in visiblePoints)
             {
                 //Rotates to look at angle
                 while (Vector3.Angle(transform.right, item.direction) > 3)
                 {
                     transform.right = Vector3.RotateTowards(transform.right, item.direction, Time.deltaTime * rotSpeed, 0);
                     yield return null;
                 }
 
                 yield return new WaitForSeconds(1.1f);
             }
         }
     }
 
     //Reseting
     visiblePoints.Clear();
     lastKnownPos = resetPos;
     lastSuspectPos = resetPos;
     wasExecuted = false;
 }

Class code:

 public class Waypoints : IComparable<Waypoints>
 {
     public Vector3 direction;
     public float eulerZ;
     public float distance;
 
     //Gives class ability to be sorted
     public int CompareTo(Waypoints other)
     {
         if (this.eulerZ > other.eulerZ)
             return 1;
         else if (this.eulerZ < other.eulerZ)
             return -1;
         else
             return 0;
     }
 
     //Constructor
     public Waypoints(Vector3 dir, float z, float dis)
     {
         direction = dir;
         eulerZ = z;
         distance = dis;
     }
 }

I guess I'm kind of asking a lot here. Any help is great, as I don't fully understand lists or coroutines, which I suspect to the problem somehow. I'm really lost, the error is so inconsistent and I have no idea what is going on, or what the List.cs file is.