"Directory index forbidden by Options directive" in Phusion Passenger 5

You may get a 403 Forbidden error code for a Rails 2.3 application under Phusion Passenger 5. And the corresponding error message in the log is similar to Directory index forbidden by Options directive: /webapps/xxxxxx/current/public/. Well, the cause of this error may be totally unrelated to the Options directive in httpd.conf.