It may be "invalid Note ID", which is something different from a Notes ID.

If it's in Notes, stop by & have the user enable LotusScript debugging and debugging the email-send.

It normally pops out of that if it's a "Note ID". Either the reference document isn't passed through properly, or some customization of the Notes DB isn't doing particularly well for this user.

This morning, she logged in and sent an email and didn't get the pop up message, so seems to be fine now.  If it shows up again, I'll follow your steps below.