By function I mean a LabVIEW function or VI. If the paths are constants, I'd check to make sure that the path is still valid. You could also replace the constant with the File Dialog function on the File I/O>Advanced File Options palette or use a front panel path control with the browse button visible. You can click the Browse button to select a file as well. If you select a file with a known good path and don't get the error, then you'll know something is wrong with the constant.