When an error occurs in the proxies, it silently eats the exception and returns a 503 response. It really needs to log the error to the console so that its apparently that an exception occurred.

See https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/handlers/proxy/ProxyConnectionPool.java#L179

which eventually calls https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/handlers/proxy/ProxyHandler.java#L238