See https://discourse.phabricator-community.org/t/constraints-parameter-type-handling-in-maniphest-search/2636.

(The caller provided constraints={...} via cURL, but you can't mix JSON and HTTP parameters like that.)

Called curl 'http://local.phacility.com/api/maniphest.search?api.token=...&constraints=X' (with a valid API token), got a more sensible error.