Seems like the problem is reated to either a conflict between the user profile and the Excel Objects Library or it could be triggered by a Windows/Office update. For whatever reason ... that line of code simply stop working and started generating that errror. I could probably have their IT department reinstall Office 2000 and the related service packs on those machines but at this point it's not worth it.
As it turns out, the source file is actually a HTML file with a .xls extension for which Excel translates to a workbook when it opens it up. Since that was the only reason for using the Excel Objects (open/save the source file), the other option is to change the import to use a Text HTML file thus bypassing the Excel Object altogether.
This doesn't answer the question as to why that message just started showing up after 2 years but it solves the problem with importing the file.
Thanks for all the comments and replies.
ET