I have not experienced this before and move workbooks with live connections between folders on my machine all the time. A couple things to try.
1. Open Tableau Desktop and see if the workbook is in the recover this workbook prompt that appears in the later releases of Tableau Desktop. It is an auto-recovery capability that was added and it might have a version of the workbook that is not in the failed state.
2. Try moving the workbook to the original file location and opening the workbook. Does this allow the workbook to open as expected?
3. Another step to take would be try and create a new workbook in the subfolder connected to the same data source. Does this cause an error as well?
4. Try reinstalling Tableau Desktop: https://kb.tableau.com/articles/issue/internal-error-an-unexpected-error-occurred-and-the-operation-could-not-be-complet…
Hope this helps!