http://lobid.org/resource/HT016371804 does now (again) exist. That's because the resource was updated not long ago (20150221) and was than ingested regularly. Your are right that it 'wasn't there in the first place, and I am still investigating this.
http://lobid.org/resource/HT016371804 , http://okeanos-www.hbz-nrw.de/F/?func=find-c&ccl_term=IDN%3DHT016371804
http://lobid.org/resource/HT018449929 , http://okeanos-www.hbz-nrw.de/F/?func=find-c&ccl_term=IDN%3DHT018449929