So I have seen at least once where the COM port has the driver loaded up but it cannot establish the connection. I have heard that if you plug the printer into a COM port and locate the port (Lets say COM3), from there unplug the printer and the control board may still be available, from there you can uninstall the driver. Then restart the computer and the printer, once everything is back up you can start Mattercontrol then once it is up plug in the printer and turn it on. This may establish the connection for you.
We know from testing a few other slicers that the issue is with the computer so it has to be the COM port is being saved with incorrect information (that is me as a simple person explaining networking). Lars might be able to explain this one a bit better than I can.
Give it a shot and lets see what happens.