Tras muchos sudores al final he conseguido desbloquearlo y ya lo tengo con la última xiaomi.eu.
Lo único que me ha funcionado es ésto que leí en el foro de miui, os lo pego por si le sirve de ayuda a alguien más.
1. Disable signature verification in Windows and started test mode (
http://en.miui.com/thread-237673-1-1.html)
2. Remove installed driver (reboot the pc)
3. Installe PDA Driver (
http://pdanet.co/a) and connect phone during the intallation
4. Remove device from i.mi.com
5. Sign out account from device
6. Factory reset device
7. In the wizard skip the account login
8. Enable dev settings and turn on OEM Unlock, adb and check Mi Unlock status (login in this step with your account ID, NOT mail or phone number) and get again -1 error
9. login in i.mi.com and find the device
10. reboot the phone in fastboot
11. use this version of Mi Unlocker
http://en.miui.com/thread-429397-1-1.html and login also here with account ID (A mí esta versión me daba error, me funcionó con la versión MiFlashUnlock_1.1.0317.1_en)
I don't know if something of this is really necessary, but with this combo I solved. If someone would try only with that version of Mi Unlocker and report the result probably may help someone else.
Enviado desde mi MI 5 mediante Tapatalk