diff --git a/src/main/python/input_backend.py b/src/main/python/input_backend.py
index ad445bc..fc04ab7 100644
--- a/src/main/python/input_backend.py
+++ b/src/main/python/input_backend.py
@@ -3,6 +3,7 @@ from config_parse import CFG
 from datetime import datetime, timedelta
 import requests
 import os
+import localization.de as de
 
 from dbfread import DBF
 import timeutils
@@ -131,6 +132,7 @@ def processExternalData(datapoints, plotNameKey, fromTime, toTime, dtype):
             content = f.read()
 
     skipBecauseFirstLine = True
+    error = None
     for l in content.split("\n"):
         if not ";" in l:
             continue
@@ -152,8 +154,10 @@ def processExternalData(datapoints, plotNameKey, fromTime, toTime, dtype):
             datapoints[plotNameKey].data  += [float(cleanFloat)]
             datapoints[plotNameKey].times += [timestamp]
         except ValueError as e:
-            print(l)
-            raise e
+            error = de.warning_ext_data
+
+    # return a warning indication for qt #
+    return error
 
 
 def read_in_file(path, backend=None, outsideData=False, plotOutsideTemp=True, plotOutsideHum=True):
@@ -213,15 +217,22 @@ def read_in_file(path, backend=None, outsideData=False, plotOutsideTemp=True, pl
                 raise NotImplementedError("Cannot determine filetype, cannot continue. Exit.")
         
         # if nessesary download and process external data #
+        error = None
         if outsideData:
 
             fromTime = datapoints[CFG("plot_temperatur_key")].getFirstTime()
             toTime   = datapoints[CFG("plot_temperatur_key")].getLastTime()
 
-            processExternalData(datapoints, pto, fromTime, toTime, CFG("dtype_temperatur"))
-            processExternalData(datapoints, pho, fromTime, toTime, CFG("dtype_humidity"))
+            error1 = processExternalData(datapoints, pto, fromTime, toTime, CFG("dtype_temperatur"))
+            error2 = processExternalData(datapoints, pho, fromTime, toTime, CFG("dtype_humidity"))
 
-        return datapoints
+            # pass on warnings #
+            if error1:
+                error = error1;
+            else:
+                error = error2
+
+        return (datapoints, error)
 
 def dbfread(path,datapoints,pt,ph,pd):
         for record in DBF(path):
diff --git a/src/main/python/localization/de.py b/src/main/python/localization/de.py
index 0c0307b..3fad5b6 100644
--- a/src/main/python/localization/de.py
+++ b/src/main/python/localization/de.py
@@ -20,3 +20,6 @@ start_section       = "Ausführen"
 close               = "Schließen"
 open_pic            = "Bild öffnen.."
 bad_time            = "Fehlerhafte Zeitangabe!"
+warning             = "Warnung"
+button_idc          = "Egal, weiter!"
+warning_ext_data    = "Fehlerhafte oder fehlende Werte in externen Daten für angegebenen Zeitraum."
diff --git a/src/main/python/main.py b/src/main/python/main.py
index 864e001..045190a 100755
--- a/src/main/python/main.py
+++ b/src/main/python/main.py
@@ -153,11 +153,18 @@ class WidgetGallery(QDialog):
 
         # workaround for checkboxes changed #
         outsideDataNeeded = self.boxOTemp.isChecked() or self.boxOHumidity.isChecked()
-        self.datapoints = input_backend.read_in_file(self.srcFileString, 
+        self.datapoints, error = input_backend.read_in_file(self.srcFileString,
                                                 outsideData=outsideDataNeeded,
                                                 plotOutsideTemp=self.boxOTemp.isChecked(),
                                                 plotOutsideHum=self.boxOHumidity.isChecked())
 
+        if error:
+            errorBox = QMessageBox(self)
+            errorBox.setAttribute(PyQt5.QtCore.Qt.WA_DeleteOnClose)
+            errorBox.setText(self.localization.warning)
+            errorBox.setDetailedText(error)
+            errorBox.show()
+
         # build dates #
         try:
             startTimeHelper = dt.datetime.strptime(self.startTimeEdit.text(),"%H:%M")
@@ -212,7 +219,7 @@ class WidgetGallery(QDialog):
         waitDialog.setText(self.localization.wait_dialog_text)
         waitDialog.show()
         try:
-            self.datapoints = input_backend.read_in_file(self.srcFileString, 
+            self.datapoints, error = input_backend.read_in_file(self.srcFileString,
                                                     outsideData=False,
                                                     plotOutsideTemp=False,
                                                     plotOutsideHum=False)