Did you create that VI in LabVIEW 5.1, or did you save it for previous from LabVIEW 6.0? Can you still open it with LabVIEW 6.0?
If you post the VI, we might be able to troubleshoot what is wrong with it.
This error means that you changed the type of a control into something that is not supported in LabVIEW 5.1. What was the last change you made to this VI before you started getting the error? Hopefully you have a backup copy from which you can recover your work.
Zvezdana S.