You may want to check the system control manager/system/devices page for installed serial devices. Most likely, you'll notice, that on your laptop, a considerable number of serial ports is connected to modems, bluetooth serial emulations and similar. Remember the port number assigned to your serial adapter and try to access it with the correct port. In some cases, double port assignment may be an issue. Usually you can adjust the virtual com port assignment under serial adapter advanced settings, if necessary.
P.S.: Redaing your last post, conflicting port assignments may exist. You also should check for possible tuning settings for the serial adapter, e.g. latency. FTDI has it, I'm not sure with Prolific.
If all doesn't help, I suggest to try a serial adapter with FTDI chip.