For the benefit of people who might stumble onto this thread, we found the specific setting that causes this to happen on some Windows 10 PC's (rectangles displaying in the Notation Window).

Access the "All Control Panel Items" screen. To get there, you can enter this path in Windows Explorer:
Control Panel\All Control Panel Items

Go to Region > Administrative, and click [Change system locale...]
There is currently a setting "[ ] Beta: Use Unicode UTF-8 for worldwide language support"



To fix the problem in Band-in-a-Box, uncheck the box, and reboot the PC.



Andrew
PG Music Inc.