Jobs keep failing when printing to the HOLD queue on our 2100 because of this error message: "The operation could not be completed because the printer queue specified is invalid or disabled." It has worked perfectly until recently. We also have a 3100 that accepts jobs without issue.
I think something happened on the 2100's Fiery server side, but I don't know what changed.
Try starting/restarting the EFI IPP Server service on the Fiery by right-clicking it and select Start or Restart depending on its status. Even if this temporarily could help I recommend you to contact your Xerox hotline to work out a permanent solution.