Did you unplug and replug the device (Hot PNP) with keeping the COM port opened in the application program?

This kind of failure is likely to occur when the device provided by DENSO WAVE is unplugged and replugged with keeping the COM port opened in the application program.
In this case, although the COM port number does not change, the port setting has been initialized when the device is replugged, and the communication is not established.
If you unplugged and replugged the device with keeping the COM port opened, you need to reopen the COM port in the application program to continue the communication.

This initialization is based on Windows’s plug-and-play behavior.
In the Windows platform, the DENSO WAVE USB-COM driver is automatically unloaded/reloaded from/to the memory when the USB device is unplugged/replugged.
The COM port opened in the application program is broken soon after the driver is unloaded from the memory.
Accordingly, the COM port that was reloaded when replugged is stored in a different memory space and no longer available, though in appearance it has the same number of COM port.