There's few things that can really help debug VISA errors...
-NI-Spy. NI-Spy captures calls to the VISA dll and it highlights the errors in red. You can save these errors to a file for future reference.
-Explain Error. LabVIEW has a menu pick Help/Explain Error... this will bring up a dialog that asks for error codes and gives descriptions.
-VISA help files. There's a help file for VISA in C:\vxipnp\win95\nivisa and some manuals in C:\vxipnp\win95\nivisa\manuals.
It sounds like the VI might be doing a VISA open on a resource that is already open. This usually happens because the VI open is in a loop. It may be necessary to examine all sub-VI's in loops.
