diff --git a/libs/output-mapping/src/SourcesValidator/LocalSourcesValidator.php b/libs/output-mapping/src/SourcesValidator/LocalSourcesValidator.php
index 5367a054a..2e84defe8 100644
--- a/libs/output-mapping/src/SourcesValidator/LocalSourcesValidator.php
+++ b/libs/output-mapping/src/SourcesValidator/LocalSourcesValidator.php
@@ -60,7 +60,12 @@ public function validatePhysicalFilesWithConfiguration(array $dataItems, array $
             // well, we probably should care about missing write-always sources :-/
             if (!$this->isFailedJob) {
                 throw new InvalidOutputException(
-                    sprintf('Table sources not found: %s', implode(', ', $invalidSources)),
+                    sprintf(
+                        'Output mapping error: The source file %s was not found. '
+                        . 'Verify that the transformation code creates this file in the output directory, '
+                        . 'or remove the corresponding output mapping if the file is no longer needed.',
+                        implode(', ', $invalidSources),
+                    ),
                     404,
                 );
             }
diff --git a/libs/output-mapping/tests/SourceValidator/LocalSourcesValidatorTest.php b/libs/output-mapping/tests/SourceValidator/LocalSourcesValidatorTest.php
index 4af9d1b8b..2f1955468 100644
--- a/libs/output-mapping/tests/SourceValidator/LocalSourcesValidatorTest.php
+++ b/libs/output-mapping/tests/SourceValidator/LocalSourcesValidatorTest.php
@@ -33,7 +33,9 @@ public function testValidatePhysicalFilesWithManifestWithOrphanedManifest(): voi
     public function testValidatePhysicalFilesWithConfigurationWithMissingSource(): void
     {
         $this->expectException(InvalidOutputException::class);
-        $this->expectExceptionMessage('Table sources not found: "missing.source"');
+        $this->expectExceptionMessage(
+            'Output mapping error: The source file "missing.source" was not found.',
+        );
 
         $dataItems = [$this->createMock(FileItem::class)];
         $configurationSource = [$this->createConfiguredMock(
diff --git a/libs/output-mapping/tests/Writer/StorageApiLocalTableWriterTest.php b/libs/output-mapping/tests/Writer/StorageApiLocalTableWriterTest.php
index ea3bf456d..e8dddd5c5 100644
--- a/libs/output-mapping/tests/Writer/StorageApiLocalTableWriterTest.php
+++ b/libs/output-mapping/tests/Writer/StorageApiLocalTableWriterTest.php
@@ -577,7 +577,7 @@ public function testWriteTableOutputMappingMissing(): void
             ],
         ];
         $this->expectException(InvalidOutputException::class);
-        $this->expectExceptionMessage('Table sources not found: "table81.csv"');
+        $this->expectExceptionMessage('Output mapping error: The source file "table81.csv" was not found.');
         $tableQueue = $this->getTableLoader()->uploadTables(
             configuration: new OutputMappingSettings(
                 configuration: ['mapping' => $configs],
@@ -1771,7 +1771,7 @@ public function testLocalTableUploadChecksForUnusedMappingEntries(): void
         $stagingFactory = $this->getLocalStagingFactory(logger: $this->testLogger);
 
         $this->expectException(InvalidOutputException::class);
-        $this->expectExceptionMessage('Table sources not found: "unknown.csv"');
+        $this->expectExceptionMessage('Output mapping error: The source file "unknown.csv" was not found.');
         $this->getTableLoader(
             logger: $this->testLogger,
             strategyFactory: $stagingFactory,