we recently upgraded our jetty version. when we did this both of our legacy gui war files, which no one has modified in some time, stopped working correctly. I believe I hunted the root cause to the proxy (used to proxy to a restful interface on another port), any call to the proxy throws the exception:
Ah, the evolution of the spec ...
async-supported == true
async-supported == false
That's why you didn't have to specify async-supported in the past, but now you do.
Bug about this bugs.eclipse.org/410893