For exchanges where the resolved path equals the request URI, and the client connection target path equals “/”, ProxyAction creates an empty request URI.

That is, the error occurs when the following evaluates to true, within ProxyAction.run():

exchange.getResolvedPath().equals(exchange.getRequestURI()) && clientConnection.getTargetPath().equals("/")

A patch with a fix and some tests reproducing this, and at least helped me understand the issue, is attached.