Interesting errors:
I looked into the call and got 422 with:
{"message":"Validation Failed","errors":[{"field":"[:title, :dashboard_id]","code":"already_exists","message":"Filter with this title already exists.","documentation_url":"http://docs.looker.com/"},{"field":"[:name, :dashboard_id]","code":"already_exists","message":"Filter with this name already exists.","documentation_url":"http://docs.looker.com/"}],"documentation_url":"http://docs.looker.com/"}
Does it mean filters have to be uniquely named across the whole instance?