Hello Hayden,
We did some research into your question and found that this issue seems to be isolated to the port or driver not allowing java to set the mask of the port. I set up a Prolific PL2303 USB to Serial device using v4300 drivers and tried to reproduce the issue you are seeing. I was unable to get the error to occur.
Have you tried different drivers for your USB to Serial device?
Additionally, you can reach Mango support directly at support@radixiot.com.