Evan,
That's not a LabVIEW, GPIB, or NI-VISA error code - that error is generated by the instrument driver. I went ahead and downloaded this driver from keithley.com to take a look at this Perform DC VI.vi function's source. But unfortunately, the heart of the Perform DC VI function is a "Call Library Node" call to a keithley DLL, so that is a black box. Also, no c-source included that I could find, so I could not look at that either. And since I don't have a Keithley 2400 to play with, that brings me to the end of the rope on my system.
A couple of more things you could try, however, since you actually have the instrument.
(1) Definately build a simple VI to eliminate other aspects of your program. In this case, the most simple
VI would be an Init, Perform DC VI.vi, and then a close. Make sure this simple program reproduces the error. If it doesn't, then the problem is likely introduced in your code, not the driver.
(2) If #1 reproduces the problem, then check that you have all the driver pre-requisites correct (ie, that NI-VISA is installed, that the GPIB board is working properly, etc).
(3) If #1 and #2 do not result in any discoveries, then you will have to enter the black box of that DLL. Primary method to do that, to be honest, is to contact Keithley and ask for help, since they developed the driver. Second method is to use our NI-SPY utility, which can capture all calls to VISA drivers, (since this is a VXIpnp driver, it should be fundamentally using VISA within the DLL). Within NI-SPY, you will be able to see the individual function calls, and which ones are returning errors. That may help point in the right direction.
Regards,
Greg Caesar
National Instruments
Applications Engineer