I'm not sure; it works as expected here (completes successfully). (Be aware that the HTML log in this scenario will not include the nested child project's log details [the pre-defined stylesheet only includes immediate step children of the top-level build element]).
You might try deleting the existing log file and running it again. If that doesn't help, could you send:
1) The info from Help | About | Install Info
2) Parent.log as it exists when the error occurs
3) The VBP application options file (found in Tools | File Locations | Application Options field).
To
support@kinook.com. Thanks.