You'll find that Monarch exports to brand new Excel files without difficulty, and you should find that you can modify (append/overwrite) those same Excel with Monarch without problems.
The problems start to appear when Excel users modify and save the workbooks that Monarch creates, and then you try to export from Monarch to those files.
The biggest source of problems occurs when users modify the named ranges that Monarch automatically establishes in the workbook, or the ranges that the range name refers to. That's typically when you'll see that error appear.
If at all possible, teach them to leave well enough alone.