New version of attackcti fixes the problem: "TAXII Collection object provided could not be reached. Recieved error: "406 Client Error: Not Acceptable for url https://cti-taxii.mitre.org/stix/collections/95ecc380-afe9-11e4-9b6c-751b66dd541e/". This problem was born when a new version of taxii2-client was introduced that supports TAXII 2.1 by default while MITRE's TAXII server is still on 2.0.