Windows does not allow browser to access USB devices by default. Please follow these instructions to install correct driver:

1. Download Zadig utility
2. Start the application
   
3. Connect Strato data port to PC
4. Connect Strato power port to charger or PC
5. Hold both arrows and menu button for 10 second
6. Then relase one arrow.
7. Wait one second, release all the buttons.
8. Go to Options and select List All devices
   
9. Select STM DFU in FS mode from the list
   
10. Select WinUSB
    
11. Click Replace Driver or Reinstall Driver, it will take ~1 minute
12. Continue with the bootloader update