Hi František,
Welcome to Xiaomi Customer Support!
Thank you very much for writing to us and letting us know about your concern. I am extremely sorry for the inconvenience caused, as you are unable to unlock the Bootloader.
I would request you to follow the below mentioned steps in order to fix this issue.
a. Make sure that MIUI Developer ROM has been updated.
b. Also that you have been logged in both your Phone and Mi flash unlocking tool using the common mobile phone number.
c. Once you login, enable “Find Device” option both in your PC and Phone.
d. Make sure that you have properly disabled Driver Signature Verification on your Windows system.
e. Forth, Mi PC Suite/Mi Flash Tool/Universal USB Drivers v2.0/Xiaomi USB Drivers/Xiaomi ADB Drivers v1.3/ADB USB Drivers v1.4.3
f.. Still if it is stuck on MI Logo, check below requisites :
i. Settings>>Additional Settings>>Developer Options>>Enable OEM Unlock/USB Debugging.
ii. Remove all security mechanism you have set it up including Fingerprints, password, PIN, and pattern lock. You can re-enable it again once your phone’s bootloader is unlocked.
iii. Open up any web browser on your PC. Now open Pre zobrazenie URL sa musíte prihlásiť and login using your Mi Account (the one that has been granted with unlock permission). But this time login using your User ID (UID) and not with your email or phone number.
iv. You will then see all connected device(s) there. Make sure you see only one device connected. Otherwise, you may need to firstly delete / remove other devices. Also make sure your connected device is shown online.
v. Connect your device to your computer using its USB cable and open Mi Unlock Tool.
vi. Login to Mi Unlock Tool using your / the same UID and password (not username or phone number).
vii. Finally proceed with the unlock process.
Should you have any queries, feel free to write to us. We'll be glad to assist you further.
Thank you for being a valued customer. Have a great day.