I think I'm going to have to call a halt on this. I can't help you any further with application errors, since it doesn't seem to be a VirtualBox matter. I've given my interpretation of the error message as a friendly gesture only.
Returning to the VirtualBox side of the problem, you might want to give a late model 5.x VirtualBox a try (e.g. 5.2.26), as there have been a number of bug reports concerning the serial ports since the release of 6.x.