When using the reverse proxy, url paths are encoded using the wrong type of encoding. It is using the awfully named java.net.URLEncoder which cannot be used to encode paths within a url.

https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/handlers/proxy/ProxyHandler.java#L704

What is happening is a proper url such as http://localhost:8080/foo%20bar gets sent across as http://localhost:8080/foo+bar which is incorrect.

An excellent blog post on the different types of url encoding can be found here http://blog.lunatech.com/2009/02/03/what-every-web-developer-must-know-about-url-encoding
[the only thing that post doesn't tell you is how to property encode paths in java, but I believe its as simple as something like new URI(null, null, myPath, null).getRawPath()]