It did turn out to be the type library issue after all. I asked the client to reinstall Office on the 3 computers with problems and now all run the OLE code successfully.

The info I found here on UT regarding that problem saved me dozens of hours and untold frustration--and paid for my UT subscription this year and next! Thanks to all who post their observations and findings here. You never know when your solution will be the missing piece to someone else's puzzle.