I found an error in the log:
The controller for path '/registerresult/balabala' was not found or does not implement IController.
Page URL: http://www.domain.com/registerresult/balabala
Referrer URL: /balabala
It was happened during registration, sometimes but not always.
While I'm trying to re-generate the error by registering a new account, the return url is http://www.domain.com/registerresult/1