Dirkswart, I've never referred to an external workbook that's on a drive mapped to a web URL—I didn't even know it's possible—so I've been watching this space expecting that when someone else answered your question, I'd probably learn something new. But so far no one else has responded, which usually means no one else knows either. So I'll offer a guess:

When I'm creating external references, that is, references to other workbooks, as I said they're not on the web; they're always somewhere on my hard drive, or at most on a Sharepoint drive. But I have noticed that the external reference looks like this:Then when I open the external target workbook, the reference changes automatically to But when I close it again, the reference changes back. Now, is there any chance that this is all that's happening to you, that the style of the reference is changing but it changes back later?

I don't really have much hope for that, so here's a second question: After Excel changes the look of the reference, does the reference still work?