I've also had this problem and it occurred when I tried to use the same VI on a different computer which I thought had all of the correct hardware.
Solution:
I found that the reason was because there were calls to an unrelated GPIB tool that was connected to the first computer but not the second. The problem went away when I scoured the Block Diagram and removed all of these calls. However, if you still need to use the machine then I'm sure its as simple as specifying the correct address.
