Since 2016-10-15 a redirect to a signup page has been implemented.

app/controllers/import_zendesk_controller.rb

@@ -40,6 +40,15 @@ class ImportZendeskController < ApplicationController
       return
     end
 
+    # since 2016-10-15 a redirect to a signup page has been implemented
+    if response.body =~ /(Take it for a risk-free|Take it for a risk-free)/i
+      render json: {
+        result: 'invalid',
+        message_human: 'Hostname not found!',
+      }
+      return
+    end
+
     Setting.set('import_zendesk_endpoint', "#{params[:url]}api/v2")
 
     render json: {