diff --git a/server/config/settings/base.py b/server/config/settings/base.py
index 0cbf3972..36740741 100644
--- a/server/config/settings/base.py
+++ b/server/config/settings/base.py
@@ -650,11 +650,11 @@ SENDGRID_API_KEY = env("IT_SENDGRID_API_KEY", default="")
 
 # Datatrans (payment)
 # See https://admin.sandbox.datatrans.com/MerchSecurAdmin.jsp)
-DATATRANS_HMAC_KEY = env("DATATRANS_HMAC_KEY")
+DATATRANS_HMAC_KEY = env("DATATRANS_HMAC_KEY", default="")
 
 # See https://admin.sandbox.datatrans.com/MenuDispatch.jsp?main=1&sub=4)
 #  => echo -n "Username:Password" | base64
-DATATRANS_BASIC_AUTH_KEY = env("DATATRANS_BASIC_AUTH_KEY")
+DATATRANS_BASIC_AUTH_KEY = env("DATATRANS_BASIC_AUTH_KEY", default="")
 
 if APP_ENVIRONMENT.startswith("prod"):
     DATATRANS_API_ENDPOINT = "https://api.datatrans.com"
diff --git a/server/vbv_lernwelt/shop/views.py b/server/vbv_lernwelt/shop/views.py
index 52751823..20488dec 100644
--- a/server/vbv_lernwelt/shop/views.py
+++ b/server/vbv_lernwelt/shop/views.py
@@ -114,9 +114,11 @@ def checkout_vv(request):
 
     # already initialized -> redirect to payment page again
     if checkout := checkouts.filter(state=CheckoutState.INITIALIZED).first():
-        return JsonResponse(
-            {"next_step_url": get_payment_url(transaction_id=checkout.transaction_id)}
-        )
+        # FIXME: Ask datatrans how they advice to handle this case:
+        #  - 1. Why is the payment url not reusable? Calling the payment url again
+        #       with the same transaction id results in a blank page.
+        #  - 2. After 30 minutes, the transaction is fails but our webhook is not called?
+        return JsonResponse({"next_step_url": get_payment_url(checkout.transaction_id)})
 
     # not yet initialized at all, or canceled/failed
     base_redirect_url = request.data["redirect_url"]
@@ -143,9 +145,7 @@ def checkout_vv(request):
         **request.data["address"],
     )
 
-    return JsonResponse(
-        {"next_step_url": get_payment_url(transaction_id=transaction_id)}
-    )
+    return JsonResponse({"next_step_url": get_payment_url(transaction_id)})
 
 
 def webhook_url(base_url: str) -> str: