I was able to update my phone using SEUS under Windows 8.1 x64, up to date.
I started SEUS using :
- "Solve compatibility problem" right click menu
- "Solve problem" (2nd option)
- "Application started previously under older OS"
- Selecting Windows 7 compatibility mode.
The Xperia SP re-apperared in the device list, and I was able to go through the whole update process.
Hope it will help other people to pass that frustrating moment