I had the same problem about 1 year ago. This was the answer from NI :
It turns out that when you update to NI-VISA 2.6.0, we do not actually update the visarc file in the LabVIEW 6.0x directory. We will try to fix that in a future version of our installer. For now, just copy the attached file into your LabVIEW\resource directory.