diff --git a/LehrerApp.Desktop/App.axaml.cs b/LehrerApp.Desktop/App.axaml.cs
index 9e73574..96679e2 100644
--- a/LehrerApp.Desktop/App.axaml.cs
+++ b/LehrerApp.Desktop/App.axaml.cs
@@ -112,7 +112,17 @@ public class App : Application
         }
         finally
         {
-            _serviceProvider.Dispose();
+            // Der direkte WebUntis-Client meldet seine Sitzung asynchron ab. Ein synchrones
+            // ServiceProvider.Dispose() lehnt reine IAsyncDisposable-Dienste ab und ließ die App
+            // beim Schließen mit InvalidOperationException abstürzen.
+            try
+            {
+                _serviceProvider.DisposeAsync().AsTask().GetAwaiter().GetResult();
+            }
+            catch (Exception ex)
+            {
+                AppBootstrapper.Logger.Error("Dienste konnten beim Beenden nicht vollständig freigegeben werden.", ex);
+            }
             _serviceProvider = null;
         }
     }
diff --git a/LehrerApp.WebUntis.Tests/WebUntisClientTests.cs b/LehrerApp.WebUntis.Tests/WebUntisClientTests.cs
index e6df3aa..937aff8 100644
--- a/LehrerApp.WebUntis.Tests/WebUntisClientTests.cs
+++ b/LehrerApp.WebUntis.Tests/WebUntisClientTests.cs
@@ -16,7 +16,7 @@ public sealed class WebUntisClientTests
             "2\t20\t8b\tMeier\tBerta\r\n");
         var handler = new QueueHandler(
             Json("{\"jsonrpc\":\"2.0\",\"result\":{\"sessionId\":\"session-1\"}}"),
-            Json("{\"data\":{\"finished\":true,\"reportParams\":\"foo=bar\"}}"),
+            Json("{\"data\":{\"finished\":true,\"error\":false,\"reportParams\":\"foo=bar\"}}"),
             new HttpResponseMessage(HttpStatusCode.OK)
             {
                 Content = new ByteArrayContent(latin1),
diff --git a/LehrerApp.WebUntis/WebUntisClient.cs b/LehrerApp.WebUntis/WebUntisClient.cs
index 80d1905..b54996d 100644
--- a/LehrerApp.WebUntis/WebUntisClient.cs
+++ b/LehrerApp.WebUntis/WebUntisClient.cs
@@ -417,7 +417,7 @@ public sealed class WebUntisClient : IAsyncDisposable
         }
 
         if (!TryProperty(payload, "data", out var data) || data.ValueKind != JsonValueKind.Object ||
-            (TryProperty(data, "error", out var reportError) && reportError.ValueKind is not JsonValueKind.Null and not JsonValueKind.Undefined))
+            (TryProperty(data, "error", out var reportError) && HasErrorValue(reportError)))
             throw new WebUntisException($"Report-Anfrage fehlgeschlagen: {ErrorMessage(payload, response)}");
         if (!OptionalBoolean(data, "finished")) return null;
         return ReportData.From(data);
@@ -646,6 +646,19 @@ public sealed class WebUntisClient : IAsyncDisposable
                (string.Equals(value.GetString(), "true", StringComparison.OrdinalIgnoreCase) || value.GetString() == "1");
     }
 
+    private static bool HasErrorValue(JsonElement value) => value.ValueKind switch
+    {
+        JsonValueKind.Null or JsonValueKind.Undefined or JsonValueKind.False => false,
+        JsonValueKind.True => true,
+        JsonValueKind.String => !string.IsNullOrWhiteSpace(value.GetString()) &&
+                                !string.Equals(value.GetString(), "false", StringComparison.OrdinalIgnoreCase) &&
+                                value.GetString() != "0",
+        JsonValueKind.Number => !value.TryGetInt32(out var number) || number != 0,
+        JsonValueKind.Array => value.GetArrayLength() > 0,
+        JsonValueKind.Object => value.EnumerateObject().Any(),
+        _ => true,
+    };
+
     private static string ErrorMessage(JsonElement payload, HttpResponseMessage response)
     {
         if (payload.ValueKind == JsonValueKind.String) return payload.GetString() ?? "Unerwartete Antwort.";
@@ -654,8 +667,25 @@ public sealed class WebUntisClient : IAsyncDisposable
             if (TryProperty(payload, "error", out var error) && OptionalString(error, "message") is { } errorMessage)
                 return errorMessage;
             if (OptionalString(payload, "message") is { } message) return message;
-            if (TryProperty(payload, "data", out var data) && OptionalString(data, "message") is { } dataMessage)
-                return dataMessage;
+            if (TryProperty(payload, "data", out var data))
+            {
+                if (OptionalString(data, "message") is { } dataMessage) return dataMessage;
+                if (data.ValueKind == JsonValueKind.Object)
+                {
+                    var dataFields = data.EnumerateObject().Select(property => property.Name).Take(8).ToArray();
+                    if (dataFields.Length > 0)
+                        return $"HTTP {(int)response.StatusCode}; unerwartete data-Felder: " +
+                               $"{string.Join(", ", dataFields)}.";
+                }
+                else
+                {
+                    return $"HTTP {(int)response.StatusCode}; data hat den Typ {data.ValueKind}.";
+                }
+            }
+            var fields = payload.EnumerateObject().Select(property => property.Name).Take(8).ToArray();
+            return fields.Length == 0
+                ? $"HTTP {(int)response.StatusCode}; leeres JSON-Objekt."
+                : $"HTTP {(int)response.StatusCode}; unerwartete JSON-Felder: {string.Join(", ", fields)}.";
         }
         return response.IsSuccessStatusCode ? "Unerwartete JSON-Antwort." : $"HTTP {(int)response.StatusCode}";
     }