diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ForbiddenApiException.cs b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ForbiddenApiException.cs
new file mode 100644
index 0000000..09ae1bd
--- /dev/null
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ForbiddenApiException.cs
@@ -0,0 +1,18 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 403 API exception.
+/// </summary>
+public sealed class ForbiddenApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ForbiddenApiException"/> class.
+    /// </summary>
+    /// <param name="message">Response message.</param>
+    public ForbiddenApiException(string message)
+        : base(403, MetaCode.NoAccess, message)
+    {
+    }
+}
diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/InvalidRequestApiException.cs b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/InvalidRequestApiException.cs
new file mode 100644
index 0000000..d93772c
--- /dev/null
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/InvalidRequestApiException.cs
@@ -0,0 +1,18 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 400 API exception.
+/// </summary>
+public sealed class InvalidRequestApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="InvalidRequestApiException"/> class.
+    /// </summary>
+    /// <param name="message">Response message.</param>
+    public InvalidRequestApiException(string message)
+        : base(400, MetaCode.InvalidRequest, message)
+    {
+    }
+}
diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Interfaces/IDocumentRepository.cs b/src/core/DigitalData.TaskFlow.Application/Common/Interfaces/IDocumentRepository.cs
index 058c4fd..c42220e 100644
--- a/src/core/DigitalData.TaskFlow.Application/Common/Interfaces/IDocumentRepository.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Interfaces/IDocumentRepository.cs
@@ -12,6 +12,6 @@ public interface IDocumentRepository : IRepository
     /// </summary>
     /// <param name="id">Document identifier.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
-    /// <returns>Binary document payload.</returns>
-    Task<DocumentBinaryResult> GetAsync(string id, CancellationToken cancellationToken);
+    /// <returns>Binary document payload when found; otherwise <see langword="null"/>.</returns>
+    Task<DocumentBinaryResult?> GetAsync(string id, CancellationToken cancellationToken);
 }
diff --git a/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs b/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
index 206b481..b48c961 100644
--- a/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
@@ -1,4 +1,5 @@
 using DigitalData.TaskFlow.Application.Common.Factories;
+using DigitalData.TaskFlow.Application.Common.Exceptions;
 using DigitalData.TaskFlow.Application.Common.Interfaces;
 using DigitalData.TaskFlow.Domain.Common;
 using DigitalData.TaskFlow.Domain.Documents;
@@ -45,6 +46,11 @@ public sealed class GetDocumentQueryHandler(IDocumentRepository documentReposito
     {
         var document = await documentRepository.GetAsync(request.Id, cancellationToken);
 
+        if (document is null)
+        {
+            throw new NotFoundApiException("Dokument nicht gefunden");
+        }
+
         return ApiEnvelopeFactory.Create(document);
     }
 }
diff --git a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
index 332c77b..94ebd3c 100644
--- a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
@@ -34,6 +34,8 @@ public sealed record CloseWorkflowCommand(
 public sealed class CloseWorkflowCommandValidator : AbstractValidator<CloseWorkflowCommand>
 {
     private static readonly string[] AllowedSources = ["Ferdinand", "Contracts"];
+    private const int ApproveMethodId = 2;
+    private const int RejectMethodId = 3;
 
     /// <summary>
     /// Initializes validation rules for <see cref="CloseWorkflowCommand"/>.
@@ -53,6 +55,20 @@ public sealed class CloseWorkflowCommandValidator : AbstractValidator<CloseWorkf
             .NotEmpty()
             .Must(source => AllowedSources.Contains(source))
             .WithMessage("Source must be one of the supported source systems.");
+
+        RuleFor(x => x.ClosingMethodId)
+            .Must(methodId => methodId is ApproveMethodId or RejectMethodId)
+            .WithMessage("Unbekannte closing_method_id");
+
+        RuleFor(x => x.Comment)
+            .NotEmpty()
+            .When(x => x.ClosingMethodId == RejectMethodId)
+            .WithMessage("Kommentar ist fur diese closing_method Pflicht");
+
+        RuleFor(x => x.RejectionReasonId)
+            .NotNull()
+            .When(x => x.ClosingMethodId == RejectMethodId)
+            .WithMessage("rejection_reason_id ist fur diese closing_method Pflicht");
     }
 }
 
@@ -79,6 +95,16 @@ public sealed class CloseWorkflowCommandHandler(IWorkflowRepository workflowRepo
             decision,
             cancellationToken);
 
+        if (result.Outcome == CloseWorkflowOutcome.NoAccess)
+        {
+            throw new ForbiddenApiException("Kein Zugriff");
+        }
+
+        if (result.Outcome == CloseWorkflowOutcome.NotImplemented)
+        {
+            throw new InvalidOperationException("Closing integration is not finalized yet.");
+        }
+
         if (result.ProcessId <= 0)
         {
             throw new NotFoundApiException("Vorgang nicht gefunden");
diff --git a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Queries/GetWorkflows/GetWorkflowsQuery.cs b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Queries/GetWorkflows/GetWorkflowsQuery.cs
index 78a3e16..aa4449e 100644
--- a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Queries/GetWorkflows/GetWorkflowsQuery.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Queries/GetWorkflows/GetWorkflowsQuery.cs
@@ -45,6 +45,11 @@ public sealed class GetWorkflowsQueryHandler(IWorkflowRepository workflowReposit
     {
         var data = await workflowRepository.GetAsync(request.WisagId, cancellationToken);
 
+        if (data.Sources.Count == 0)
+        {
+            return ApiEnvelopeFactory.Create(data, MetaCode.NoAccess);
+        }
+
         var hasListingData = data.Sources
             .SelectMany(source => source switch
             {