I am not exactly sure why you are getting the error either. The links I found on this are
LabVIEW.LIB Error When Calling a LabVIEW DLL Built with VIs That Use External CodeI am assuming that all the code you are calling has been mass compiled into 6.0.2. One way to ensure that would be to create a application distribution and then mass compile that VI. That way you are assured that all the VIs are 6.0.2. Once you have done that you can re-create the dll.
One question I had was if there was a specific reason you needed the dll to be in 6.0.2, especially beacuse you have LabVIEW 7.0.
Also what exactly does the VI do? Are you doing data acquisition, gpib, or
maybe Serial? If so are you using current versions of the drivers for DAQ or GPIB? Are you using the old serial VIs or VISA VIs for the serial? If not on any of these I would update them and see if that helps.
Sorry if none of this helps.