Fix NFO creation 500 error for missing folders

- Auto-create series folder if it doesn't exist
- Add unit and integration tests for folder creation
- NFO creation now works for newly added series

docs/instructions.md

@@ -119,10 +119,32 @@ For each task completed:
 
 ## TODO List:
 
-All tasks completed! The NFO database query issue has been resolved.
+All tasks completed! Recent issues have been resolved.
 
 ## Recently Fixed Issues:
 
+### ✅ Fixed: NFO Creation 500 Error - Missing Folder (2026-01-18)
+
+**Issue:** POST http://127.0.0.1:8000/api/nfo/blue-period/create returns 500 (Internal Server Error) with message "Series folder not found: /home/lukas/Volume/serien/Blue Period"
+
+**Root Cause:** The NFO service was checking if the series folder existed before creating NFO files, and raising a FileNotFoundError if it didn't. This prevented users from creating NFO files for newly added series where no episodes had been downloaded yet.
+
+**Solution:** Modified `create_tvshow_nfo()` in `nfo_service.py` to automatically create the series folder (with `mkdir(parents=True, exist_ok=True)`) if it doesn't exist, instead of raising an error.
+
+**Files Modified:**
+- [src/core/services/nfo_service.py](../src/core/services/nfo_service.py)
+
+**Tests Added:**
+- [tests/unit/test_nfo_service_folder_creation.py](../tests/unit/test_nfo_service_folder_creation.py) - Unit test verifying folder creation
+- [tests/integration/test_nfo_folder_creation.py](../tests/integration/test_nfo_folder_creation.py) - Integration test for end-to-end verification
+
+**Verification:** 
+- Unit and integration tests pass
+- NFO files can now be created for series even when the folder doesn't exist
+- Folder is automatically created when needed
+
+---
+
 ### ✅ Fixed: NFO Service 503 Error (2026-01-18)
 
 **Issue:** Failed to load resource: the server responded with a status of 503 (Service Unavailable) when creating NFO files.
@@ -132,6 +154,7 @@ All tasks completed! The NFO database query issue has been resolved.
 **Solution:** Updated `fastapi_app.py` startup code to sync NFO configuration (including TMDB API key) from `data/config.json` to `settings` object, similar to how `anime_directory` was already being synced.
 
 **Files Modified:**
+
 - [src/server/fastapi_app.py](../src/server/fastapi_app.py)
 
 **Verification:** NFO endpoints now return 200 OK instead of 503, and NFO creation is functional.