When the OAI-PMH harvester in DSpace encounters an error during harvesting, it displays only a generic error in the UI:
Unknown error occured while generating an OAI response on 2014-12-17 23:45:37.82
It logs the specific error message and stacktrace in dspace.log, e.g.:
ERROR org.dspace.harvest.OAIHarvester @ Error occured while generating an OAI response: Server returned HTTP response code: 400 for URL: http://localhost:8080/xmlui/bitstream/123456789%2F15/1/bitstream null
Such behaviour doesn't help the DSpace manager, who might not have access to DSpace logs, see what the problem is. UI should display the specific error message.