Maybe I'm just exposing my ignorance, but these users who are having the problem—do they get the "Debug"-button option? Because if they do, you can have them hit that button, then tell you what statement the macro bombed on. That, at least, would narrow it down for you.

If that button isn't offered, all that occurs to me is a) to go to one of their machines yourself, personally, and run and debug it there, or b) to select a reasonably savvy victim who's having the problem and seems up to learning a little; have that person put breakpoints at different places in the program and run it experimentally, narrowing the search until you've zeroed in on the place that's causing the problem. Oh, wait, you said this problem was occurring when they open the workbook, so no setting breakpoints. So they'd have to insert either Stop statements or MsgBox calls to accomplish the same thing. It's a pain, but it's at least guaranteed to get you eventually to the source of the problem.