One thing I forgot to mention which happened to me and might be affecting other people too when flashing the XperiaX device;

There was no way I could make the device start in fastboot mode, nope, whether holding volume-UP or volume-DOWN or whichever combination of any of the buttons whatever!
I got the device to start up with the green LED light up but it was not recognized by fastboot!

What I could do, however was to boot it up normally, and then do "adb reboot-bootloader" which puts it correctly into fasboot mode.

Go figure.