Behind a reverse proxy a requests may fail with this error
In Jenkins 2.0 Jetty was upgraded from 8.1 to 9.2 which removes AJP functionality, and also requires that if the HOST: field is used it can not be empty.
Current winstone version includes Jetty server 9.2.15.v20160210.
The Jetty version embedded in winstone must be upgraded to have a fix
The workaround is to take care to never send an empty HOST