I had the same problem with the USB Serial Adapter where it cannot open port. Since i had a USB to USB OTG adapter from a tablet, I resorted to the USB flash technique, which was so easy. Copy dsp.hex to flash drive, then insert into inverter. While inverter is running normally, press top left button for 3 seconds, then press top left button again to choose upgrade, then press middle left to choose yes. Then wait like 10min, there is no progress indicator, inverter will reboot when it done. If it doesn't then press top left button again, and wait. I believe you have to make sure your Remote unit is running at least 2.2.
I used this method for a Mecer King and two Kodak Kings in parallel.
When i flashed a Kodak King, I initially got error 32 (communication interrupted) after it rebooted, I then reflashed using USB again, and it then worked.