REST API, only in the area of deployment, is not backward compatible from 6.0 to 5.x; the endpoint you want is now
But you should not be getting this error from WebUI, though. Does this WebUI belong to a 5.x instance? Please file a Support case, and provide details about all the Splunk nodes involved, with respective versions.
I'm going from 5.0.4 to 6.0. I'm not sure if the extract was interrupted on the first run, but I extracted 6.0 again on top of what I had and the correct page shows up now. Although the Forwarder Management page would not work until I removed my existing serverclass.conf file and start from scratch. My endpoint now shows manager/system/deploymentserver#deploymentserverclasses