I would recommend checking your DSpace log files (on the backend) for any specific errors relating to that harvest request. Unfortunately, it's impossible to provide help without seeing a more specific error as to why there was an "Error while harvesting using setSpec 'com_123456789_719". Finding the exact error might require reproducing this error from OAI Harvesting and then checking the logs.
Once you find the more detailed error message, you can send it to this list for further advice.
Tim