Unable to load /Find/proxy/ - returns 406 Not acceptable

Getting a wierd response from Episerver Find UI; all requests, bot POST and GET to endpoints beginning with /Find/proxy/ returns a 406 (Not acceptable) header. Both in development and production.

Console dump:

GET http://some-site-dev.local/secret-path-ui/Find/proxy/_admin/unifiedweights/?tags=searchtype%3Adefault&dojo.preventCache=1463639710173 406 (Not Acceptable)
{message: "Unable to load /secret-path-ui/Find/proxy/_admin/unifiedwe…fault&dojo.preventCache=1463639710173 status: 406", response: Object, status: 406, responseText: "", xhr: XMLHttpRequest}
GET http://some-site-dev.local/secret-path-ui/Find/proxy/_admin/unifiedweights/?tags=searchtype%3Adefault&dojo.preventCache=1463639710289 406 (Not Acceptable)
POST http://some-site-dev.local/secret-path-ui/Find/proxy/_search 406 (Not Acceptable)
{message: "Unable to load /secret-path-ui/Find/proxy/_search status: 406", response: Object, status: 406, responseText: "", xhr: XMLHttpRequest}

Version list:

• EPiServer.Find 11.1.6.4396
• EPiServer.Find.Cms  11.1.6.4396
• EPiServer.Find.Framework 11.1.6.4396
• Episerver.CMS.UI 9.6.0
• Episerver.CMS.UI.Core 9.6.0

Any ideas?