Another user who was unable to flash new firmware on his Duet WiFi and reported a similar problem running Bossa 1.8 managed to flash the firmware using the command line version of Bossa 1.8. I had suggested the following command line to him:
bossac.exe –port=COM_nn_ -U true -e -w -v -b C:\path\FirmwareFile.bin -R
If necessary, put in the full path at the beginning to the version of bossac.exe installed with Bossa 1.8. Substitute your own COM port number and firmware path and file name in this command.