ActiveWorkbook without a qualifying reference is the same as Application.ActiveWorkbook - the active workbook in the Excel instance that the code is running in.

If the workbook is open in the other instance of Excel, then you would refer to it as xlApp.ActiveWorkbook.

As I said, I don't use Access, so I don't know where the workbook is open.

When the code breaks, set xlApp.visible = false in the Immediate window; do you see an open workbook?