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A couple of things to explore:
I have had issues with serial devices that had to be in the 1-4 port number range to work, and finding that my usb device automatically set itself much higher. Advanced settings in windows can sort that out.
Also, there is a possibility that your serial ports are operating at a voltage too low for the device. RS232 doesn't have a fixed standard for voltage and it can commonly vary from 6v to 12v (and potentially even higher). This could be why some things work and others don't.
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