Seems this could fix: Supervisor#640

Author: lukeweber

supervisor/web.py

@@ -427,7 +427,7 @@ def render(self):
                     return NOT_DONE_YET
                 if message is not None:
                     server_url = form['SERVER_URL']
-                    location = server_url + '?message=%s' % urllib.quote(
+                    location = server_url + "/" + '?message=%s' % urllib.quote(
                         message)
                     response['headers']['Location'] = location