diff --git a/mlair/run_modules/post_processing.py b/mlair/run_modules/post_processing.py
index d4a3c4f012c8499e24f50256bd0d77c33c8776d6..6b3895f3538379ab0a5faed87de072a711c17d5f 100644
--- a/mlair/run_modules/post_processing.py
+++ b/mlair/run_modules/post_processing.py
@@ -221,8 +221,9 @@ class PostProcessing(RunEnvironment):
         for station in all_stations:
             # test data
             external_data = self._get_external_data(station, self.forecast_path)
-            if external_data is not None:
-                pass
+            if external_data is None:
+                logging.info(f"skip calculate_block_mse for {station} as no external_data are available")
+                continue
             # competitors
             if evaluate_competitors is True:
                 competitor = self.load_competitors(station)
diff --git a/mlair/run_modules/pre_processing.py b/mlair/run_modules/pre_processing.py
index 41d863a1cec5b86487374e08655c0c86815f3417..64d1bfa20a81c11b3aca79c74c057e06d0b510b8 100644
--- a/mlair/run_modules/pre_processing.py
+++ b/mlair/run_modules/pre_processing.py
@@ -428,7 +428,8 @@ class PreProcessing(RunEnvironment):
 
         if check_nested_equality(self.data_store._store, snapshot._store, skip_args=excluded_params) is True:
             self.update_datastore(snapshot, excluded_params=remove_items(excluded_params, ["transformation",
-                                                                                           "data_collection"]))
+                                                                                           "data_collection",
+                                                                                           "stations"]))
         else:
             raise ReferenceError("provided snapshot does not match with the current experiment setup. Abort this run!")