UnityWebRequest returns different errors for right urls

Question

I'm using UnityWebRequest to download asset bundles from my server. Sometimes the following code returns different exceptions such as Failed to receive data , Cannot resolve destination host, Cannot connect to destination host, Unknown Error, Request timeout and so on.

Here is the part of my code.

UnityWebRequest www = UnityWebRequestAssetBundle.GetAssetBundle(url,(uint)data.version,0);

                    Debug.Log ("Downloading " + url);
                    
                    yield return www.SendWebRequest();
                    AssetBundle bundle = null;
                    if(www.isNetworkError || www.isHttpError) {
                        Debug.Log(www.isNetworkError);
                        Debug.Log(www.isHttpError);
                        Debug.Log(www.error);

                        if (ARSceneManager.Instance != null)
                            data.downloadAssetBundlesCompleted?.Invoke(null);
                    } else {
                        bundle = DownloadHandlerAssetBundle.GetContent(www);
                    }

                    if (!string.IsNullOrEmpty(www.error)) {

                        throw new Exception("WWW download: " + url + ", " + www.error);
                    }

So I get the errors for different urls and in some cases.

P.S. the same URL works for the next time, also it works in browser too. So I don't think there are issues with urls.

Thanks.