do not send a useless json respond back, as it is not used on the client side anyway

app/controllers/volunteers_controller.rb

@@ -82,7 +82,7 @@ class VolunteersController < ApplicationController
   def update_waive_and_iban
     authorize @volunteer
     @volunteer.update(volunteer_params)
-    respond_to :json
+    render json: nil, status: :ok
   end
 
   def terminate

app/views/volunteers/update_waive_and_iban.json.jbuilder

-json.volunteer do
-  json.id @volunteer.id
-  json.waive @volunteer.waive
-  json.iban @volunteer.iban
-  json.bank @volunteer.bank
-  json.errors @volunteer.errors&.messages
-end