Open Bootloader - "Couldn't Verify Device" without error

We found a great description on the English forum, which might help you to open it. It is strongly recommended to use the EMAIL address of the Mi account, not a phone number or ID, an EMAIL address!!!(I mean MANDATORY!!!)

Let's go through the steps step by step:

0. Completing your account English forum. Mi Pc Suite and MiFlash programs must be installed!

1. Request an opening on this page website (This can take up to 2-3 weeks!)

2. Once your application is successful, you must download the opening programme(You may not be notified when your application is accepted, so you will have to try daily after 2 weeks)

3. The unlocking process can be done with any of the FACTORY DEVELOPER ROMs, some have done it with Chinese, some with Global, the main thing is to have a developer/dev rom. (If you got your phone this way, you have no choice but to update to developer with the Updater application, if you have a webshop, you have to install the factory fastboot rom with MiFlash)

4. Log in on the phone with the access code you used to request the opening. IMPORTANT! As I said with EMAIL address! Turn on Find my device in your My Account

5. Log in to the http://i.mi.com on the EMAIL ADDRESS page! A verification code will be sent by SMS, it may not come the first time, but it will always come after 1-2 tries! After entering the code, your phone is already connected to your account. You can check this by clicking on "X connected" on the left side of the line above.

6. WAIT at least one day before the opening, just in case!!!

7. Open the Unlock program and login with the same EMAIL ADDRESS you used to request the unlock!!!

8. Turn off your phone, then start it in fastboot mode (Volume Down + Snap) until you see the bunny tinkering with the robot. Plug your phone into your computer.

9. Click on the Unlock button

Tádáááááááááááááá ennyi volt!

Source: http://en.miui.com/thread-280561-1-1.html