[Redmi K20 / Xiaomi Mi 9T Questions & Answers] Sony98:
FINALLY it worked YES :D:D:D

FIRST: The Story for everybody who has the same problem like me:

Device: Xiaomi Mi 9T

Firmware: MIUI 10 GLOBAL

Antirollbackprotectioncounter: 1

Bootloader: Unlocked

Story:

At the beginning, everything was working, but i wanted to install a custom rom (STAGOS)

For this step, i had to Update my Miui firmware on 11.0.4.0.

But this step didnt work so well via device OTA (error), then i decided to install it via custom recovery.

I still had the stock recovery on my device, so i went in fastboot mode, and installed Orangefox recovery via ADB and Fastboot on Windows. That worked without problems.

In Orangefox, i wiped as sayed in instructions data, cache and System. then i rebooted.

But before realised that was the wrong way, i stucked in fastboot.

No tools helped. ADB and Fastboot didnt work as seen in screenshots.

I tried everything via fastboot....

fastboot flash recovery XXX.img

fastboot boot XXXX.img

Fastboot format.....etc

But NOTHING worked. Everytime there were the following errors:

Target didnt report Max download size

Error: Cannot load recovery

No such file or directory

Error: Download size is higher that allowed

Failed to load boot image (Load error)

Command write failed (no error)

and some other ones too.....

It looked like, some flashes worked and some not. Sometimes it worked, sometimes not.

After some new tries it just showed via the command: Fastboot devices

????????? fastboot

That problem has been solved. Just unplug the device and wait for 10 minutes until it shuts down automatically.

If that doesnt work, you can try to replug it to pc some times again.

It looked like replugging and using different USB Port (one user solved the problem via USB 2.0) helps to get a better connection and prevent errors.

Then i tried some tools, for example android all in one tool, but no one worked.

After this, i reinstalled the driver and installed MI Flash tool (https://mi-globe.com/download-xiaomi-mi-flash-tool-all-versions/)

It looks like there are some mi flash tools around here which are not working right, but THIS one worked for me.

After installing, i connected my device, choosed the fastboot firmware (.tgz), unpacked it and choosed the right location in mi Flash tool.

Firmware files are find here: https://c.mi.com/oc/miuidownload/detail?guide=2

(You should check you Antirollback counter before to avoid a brick. Here you find some informations: https://www.xda-developers.com/xiaomi-anti-rollback-protection-brick-phone/)

DONT FORGET TO CHOOSE IN THE RIGHT DOWN CORNER: CLEAN ALL !!! (normally, clean all and lock is choosen, but that bricks your device)

After doing this, i started flashing, but there was an error: Antirollbackcheck error.

I solved it via moving the mi flashtool directly into my C hard drive, and deleted some lines in the flashall.bat, as seen in post history here.

Then....it worked for some seconds longer, but than: Error: Failed to flash dtbo.img

So i tried it via ADB and Fastboot. But nothing changed.

Then i tried flashing via mi Flash tool again, and this time the following error comes: Failed to flash system.img.

After this is thought: hmmm....something is very wrong with this device....lets try again, after a device restart.

So i tried again, let it running for ~360 seconds, and IT WORKED!

At the End: there was the following error: Error: Not catch checkpoint.

But this error (you can also see in a screenshot) it not necessary.

This is the result of removing some lines in flashall.bat.

After this my phone successfully booted im Miui 11.