NavMesh Agent won't move

Hello there!

I know, there is lots of these question out there, but I just don't find any cause for this problem:

The NavMesh agent won't move at all! As you see in the image, everything seems to be calculated correctly, but nothing happens. Here is the GameObject of the Agent:

Also, this is the attached script:

 using UnityEngine;
 using System.Collections;
 
 public class TestNav : MonoBehaviour {
 
         // Use this for initialization
         void Start () {
             GetComponent<NavMeshAgent>().destination = GenerateRandomPosition();
         }
         
         // Update is called once per frame
         void Update () {
         
         }
     
         Vector3 GenerateRandomPosition()
         {
             Vector3 ret = new Vector3();
             ret.x = Random.Range(-15f, 15f);
             ret.z = Random.Range(-300f, 300f);
             ret.y = 0.5f;
             return ret;
         }
     }

Does anybody know what the problem is?