503 Service unavaible on Android/MAUI with HttpClient

im working with MAUI to create my App. Sometimes my endpoint throws "503 Service unavaible" on my App while this is reachable on other systems.

HttpClient _client = new HttpClient();
_client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", _ud.sess_id);

string endpoint= "https://............."
            var response = await _client.GetAsync(endpoint);
            var r = await response.Content.ReadAsStringAsync();

            if (response.StatusCode != System.Net.HttpStatusCode.OK)
            {
                await DisplayAlert("Info", r, "OK");
                return;
            }

The problem stays for a day on all Android phones, then it works fine for days. On IOS there is no Problem with the same code.

Server is up and good.

Is this a known Problem and how to solve it?

Source: View source