You may be using a GPIB-ENET as JLS suggests, but error 28 in software (LabVIEW) is an EPWR error according to NI-488.2 help under the topic "iberr error codes and solutions". The EPWR error happens when a system goes into standby mode with GPIB handles open, and then you try to use those handles after a resume. So, if you could prevent that from happening I bet you'll resolve your error 28.
Scott B.
GPIB Software
National Instruments