diff --git a/Modules.Interfaces/ZUGFeRDInterface.vb b/Modules.Interfaces/ZUGFeRDInterface.vb
index d324fa15..fbdd34aa 100644
--- a/Modules.Interfaces/ZUGFeRDInterface.vb
+++ b/Modules.Interfaces/ZUGFeRDInterface.vb
@@ -18,6 +18,7 @@ Public Class ZUGFeRDInterface
         NoValidFile
         NoZugferd
         NoValidZugferd
+        MissingProperties
     End Enum
 
     Public ReadOnly Property FileGroup As FileGroups
@@ -130,7 +131,7 @@ Public Class ZUGFeRDInterface
             Dim oFoundResult As PDFAttachments.AttachmentResult = Nothing
 
             For Each oResult In oResults
-                If oResult.FileName = PDFAttachments.ZUGFERD_XML_FILENAME Then
+                If oResult.FileName.ToUpper() = PDFAttachments.ZUGFERD_XML_FILENAME.ToUpper() Then
                     oFound = True
                     oFoundResult = oResult
                 End If
diff --git a/WEBSERVICES/ZUGFeRDRESTService/Controllers/ValidationController.cs b/WEBSERVICES/ZUGFeRDRESTService/Controllers/ValidationController.cs
index 91ff5730..90565c26 100644
--- a/WEBSERVICES/ZUGFeRDRESTService/Controllers/ValidationController.cs
+++ b/WEBSERVICES/ZUGFeRDRESTService/Controllers/ValidationController.cs
@@ -8,6 +8,8 @@ using Microsoft.AspNetCore.Mvc;
 using Microsoft.Extensions.Options;
 using DigitalData.Modules.Interfaces;
 using DigitalData.Modules.Logging;
+using static DigitalData.Modules.Interfaces.Exceptions;
+using static DigitalData.Modules.Interfaces.ZUGFeRDInterface;
 
 namespace ZUGFeRDRESTService.Controllers
 {    
@@ -39,6 +41,20 @@ namespace ZUGFeRDRESTService.Controllers
                 message = String.Empty;
                 errors = new List<string>();
             }
+
+            public ValidationResponse(string Status, string Message)
+            {
+                status = Status;
+                message = Message;
+                errors = new List<string>();
+            }
+
+            public ValidationResponse(string Status, string Message, List<String> Errors)
+            {
+                status = Status;
+                message = Message;
+                errors = Errors;
+            }
         }
 
         public ValidationController(ILogging logging, IDatabase database)
@@ -61,50 +77,78 @@ namespace ZUGFeRDRESTService.Controllers
         }
 
         /// <summary>
-        /// POST: api/Validation
+        /// POST: /api/Validation
         /// </summary>
         /// <param name="files">This parameter's name needs to correspond to the html form's file-input name</param>
         [HttpPost]
         public async Task<ValidationResponse> Post(IFormFile file, string user_id)
         {
-            string oFilePath;
-            string oFileName;
+            _logger.Debug("Start processing request to ValidationController");
 
-            oFilePath = Path.Combine(Path.GetTempPath(), file.FileName);
-            oFileName = file.FileName;
+            string oFilePath = Path.Combine(Path.GetTempPath(), file.FileName);
+
+            _logger.Debug("Saving file to temp-path [{0}]", oFilePath);
 
             using FileStream oStream = new FileStream(oFilePath, FileMode.Create);
             await file.CopyToAsync(oStream);
 
+            // TODO: Delete temp file / Do we even need the temp file?
+
+            _logger.Debug("File successfully saved!");
+
             CrossIndustryDocumentType oDocument;
+            PropertyValues.CheckPropertyValuesResult oResult = new PropertyValues.CheckPropertyValuesResult();
 
             try
             {
+                _logger.Debug("Extracting ZUGFeRD Data from file [{0}]", oFilePath);
+
                 oDocument = _zugferd.ExtractZUGFeRDFileWithGDPicture(oFilePath);
-            }
-            catch (Exception ex)
-            {
-                _logger.Error(ex);
-                return new ValidationResponse()
-                {
-                    status = RESPONSE_ERROR,
-                    message = "The uploaded file is not a valid ZUGFeRD Invoice!"
-                };
-            }
 
-            try
-            {
-                PropertyValues.CheckPropertyValuesResult oResult = _props.CheckPropertyValues(oDocument, _propertyMap, "MESSAGEID");
+                _logger.Debug("Checking ZUGFeRD Data against the database");
 
-                if (oResult.MissingProperties.Count > 0)
-                {
-                    return new ValidationResponse()
-                    {
-                        status = RESPONSE_ERROR,
-                        message = "The uploaded file is a valid ZUGFeRD Invoice but missing some properties!",
-                        errors = oResult.MissingProperties
-                    };
+                oResult = _props.CheckPropertyValues(oDocument, _propertyMap, "MESSAGEID");
+
+                _logger.Debug("Result of checking against the database: {0} valid properties, {1} missing properties", oResult.ValidProperties, oResult.MissingProperties);
+
+                if (oResult.MissingProperties.Count > 0) {
+                    throw new ZUGFeRDExecption(ZUGFeRDInterface.ErrorType.MissingProperties,
+                        "Die hochgeladene Datei ist eine gültige ZUGFeRD-Rechnung, allerdings fehlen benötigte Daten.");
                 }
+
+                return new ValidationResponse()
+                {
+                    status = RESPONSE_OK,
+                    message = "Die hochgeladene Datei ist eine gültige-ZUGFeRD Rechnung"
+                };
+            }
+            catch (ZUGFeRDExecption ex)
+            {              
+                _logger.Error(ex);
+
+                // Determine which message should be sent in the response
+                string oMessage = ex.ErrorType switch
+                {
+                    ErrorType.NoValidFile => "Die hochgeladene Datei ist keine gültige Datei.",
+                    ErrorType.NoZugferd => "Die hochgeladene Datei ist keine ZUGFeRD-Rechnung.",
+                    ErrorType.NoValidZugferd => "Die hochgeladene Datei ist keine gültige ZUGFeRD-Rechnung.",
+                    ErrorType.MissingProperties => "Die hochgeladene Datei ist eine gültige ZUGFeRD-Rechnung, jedoch fehlen einige Metadaten",
+                    _ => "Die hochgeladene Datei kann nicht validiert werden.",
+                };
+
+                // Determine if any errors should be sent in the response
+                List<string> oErrors = ex.ErrorType switch
+                {
+                    // Errors contains the list of missing fields
+                    ErrorType.MissingProperties => oResult.MissingProperties,
+                    _ => new List<string>()
+                };
+
+                return new ValidationResponse()
+                {
+                    status = RESPONSE_ERROR,
+                    message = oMessage
+                };
             }
             catch (Exception ex)
             {
@@ -112,15 +156,9 @@ namespace ZUGFeRDRESTService.Controllers
                 return new ValidationResponse()
                 {
                     status = RESPONSE_ERROR,
-                    message = "The uploaded file is not a valid ZUGFeRD Invoice!"
+                    message = "Die hochgeladene Datei kann nicht validiert werden!"
                 };
-            }
-           
-            return new ValidationResponse()
-            {
-                status = RESPONSE_OK,
-                message = "The uploaded files is a valid ZUGFeRD Invoice!"
-            };
+            }           
         }
     }
 }