I do not have any insight as to the cause of this problem. However I discovered a temporary workaround that has allowed me to continue using the BOM creation tool with version 5.1.5. I went back to my last installed version of KiCad (5.1.0) and copied the \bin\python.exe file to the \bin folder of the 5.1.5 and once I did that the BOM creation scripts were working for me once again. I renamed the python.exe file that installed with version 5.1.5 rather than copying over it.
I do not have any insight as to the cause of this problem. However I discovered a temporary workaround that has allowed me to continue using the BOM creation tool with version 5.1.5. I went back to my last installed version of KiCad (5.1.0) and copied the \bin\python.exe file to the \bin folder of the 5.1.5 and once I did that the BOM creation scripts were working for me once again. I renamed the python.exe file that installed with version 5.1.5 rather than copying over it.
Thanks,
Mitch Kapa