Unity freezes when adding a GameObject to a List

Since 3 days ago Unity started freezing after adding a GameObject to a List. I haven't changed anything in that particular code, so I have no idea why this is happening:

  private IEnumerator HandleSelectedCubes() {
     if (InputManager.instance.selectedCube != null && !selectedCubes.Contains(InputManager.instance.selectedCube)) {
         if (oldSelectedCube == null) {
             selectedCubes.Add(InputManager.instance.selectedCube); // This is where it crashes
             oldSelectedCube = InputManager.instance.selectedCube;
         } else if (InputManager.instance.selectedCube.name != oldSelectedCube.name) {
             ClearSelectedCubes();
             selectedCubes.Add(InputManager.instance.selectedCube);
             oldSelectedCube = InputManager.instance.selectedCube;
             InputManager.instance.selectedCube = null;
         }

         AudioManager.instance.PlayCubeSelectSound();

         if (selectedCubes.Count == 4) {
             InputManager.instance.allowCubeSelection = false;
             moveDone = true;
             yield return new WaitForSeconds(0.07f);

             var destroySelectedCubes = true;

             foreach (var cube in selectedCubes.ToList()) {
                 if (cube != null && cube.tag == "Lightning")
                     destroySelectedCubes = false;
             }

             foreach (var cube in selectedCubes.ToList()) {
                 if (cube != null) {
                     cube.GetComponent<Cube>().hasGivenShard = true;
                     CubeAbilities.instance.TriggerSpecialCubeAbility(cube);

                     if (destroySelectedCubes) {
                         DestroyCube(cube);
                         ClearSelectedCubes();
                     }
                 }
             }

             InputManager.instance.playerMovesCount++;
             AudioManager.instance.PlayCubeDisappearSound();
             ShardManager.instance.AddSessionShards(4);
         }
     }
 }

InputManager:

   private void RegisterTouchInputs() {
         if (!IsScreenTouched() || !allowCubeSelection) return;

         var touch = Input.GetTouch(0);

         worldTouchPoint = Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position);
         var radius = 0.1f;
         var allHits = Physics2D.CircleCastAll(worldTouchPoint, radius, Vector2.zero);

         // Find closest collider that was touched
         var closestDist = Mathf.Infinity;
         GameObject closestObject = null;

         foreach (var hit in allHits) {
             // Record the object if it's the first one we check, or is closer to the touch point than the previous
             if ((closestObject == null || Vector2.Distance(closestObject.transform.position, worldTouchPoint) < closestDist) && hit.collider.transform.parent.tag != "Indestructible") {
                 closestObject = hit.collider.gameObject;
                 closestDist = Vector2.Distance(closestObject.transform.position, worldTouchPoint);
             } else {
                 return;
             }

             //If there isn't an old cube or if the touch phase has just started, select the cube
             //Or if the touch phase has started and is moving (the user is holding and selecting cubes) and previously selected cubes are the same as this one, select the cube
             if ((CubeBehavior.instance.oldSelectedCube == null || touch.phase == TouchPhase.Began ||
                 (touch.phase == TouchPhase.Moved && CubeBehavior.instance.oldSelectedCube != null && CubeBehavior.instance.oldSelectedCube.name == closestObject.name))) {
                 CubeBehavior.instance.selectedCube = closestObject;
             } //If the touch phase has started and is moving (the user is holding and selecting cubes) but previously selected cubes are the same as this one, ignore the cube
             // This is so that it doesn't break the chain if the user goes over a differently colored cube
             else if (touch.phase == TouchPhase.Moved && CubeBehavior.instance.oldSelectedCube != null && CubeBehavior.instance.oldSelectedCube.name != closestObject.name) {
                 return;
             }
         }
     }