I have an ST25i (Xperia U) that I just cannot get back to any ROM, stock or otherwise.
When I try the flashtool method (press Vol Up, then connect to computer), I get the boot splash and that’s it. The computer (windows 10) doesn’t even notice that I have plugged in anything, so I cannot install any drivers.
If I boot into recovery, I get to the TWRP recovery fine – but the computer only shows an "Unknown USB device (configuration descriptor request failed)" for which he finds no new/compatible drivers in my driver package.
Plus, the driver installer from the flashtool 0.9.14.0 does not install any drivers. No matter which I select, I always get a red croos and "could not be installed" beside it. So flashtool won’t work. 
So, I cannot get anything onto the ST25i via USB, and the phone doesn’t have an SDcard slot. How can I proceed? What is the problem here?
Help! Any help appreciated.