Very well done! There is actually already a bug report (#33559) at rt://Win32-SerialPort and the module author (or someone) has replied. The two other people who experienced the problem did what you have done with mixed success. It might not hurt to add your two bits worth in the hope that the module author can find a general solution to the issue and at least knows that a) there are people using the module, and b) people are seeing the same problem (module authors like feedback, even / especially bug reports).