Recently i wanted to flash another Wemos D1 Mini but actually had no chance to do it from my Windows 10 PC. Symptom:

failed to execute 'open' on 'serialport': failed to open serial port.

Solution 1: Downgrade the Driver of the Serial Port to an older one  as described here . Solution 2: Change the Baud-Rate of the Serial Port via the Device Manager. Right click on the USB serial port > Properties > Port setting > change the bits per second from 9600 to 115200.