Fingerprint calibration error "1515 Brightness instable"

[SnN Day :)]

Hi! I've encountered a problem when've been calibrating my sensor in CIT. My phone passes every test, but when I get to the "Put the FLESH CHART horizontally" the log shows that "KB CALIBRATION Test Failed". I searched for solutions, but the problem is that I never saw a person with "1515 Brightness instable" error. What does it mean? What should I do to fix that? I used fully black and white paper sheets. Before deleting the previous fingerprint, it could read it and showed a password screen in case prints didn't match. It cannot be a hardware issue. Of course, the all started from the moment I could not add a new sensor for replaced motherboard.

MY PHONE:
Device: Xiaomi Mi 11 Ultra (motherboard replaced, works perfectly)
The Display: ORIGINAL, FROM PURCHASED PHONE with a Hydrogel screen protector (works perfectly)
MIUI version: 14.0.2
Bootloader: Unlocked (no root)
Recovery: Stock

WHAT I TRIED:
To look at the tip of my finger to align fingerprint lines horizontally - No Effect
Complete second and third steps in a dark environment (when it needs something black and a horizontal flesh chart) - No Effect

What do you think? Can it be a screen protector or MIUI's fault?
Thanks in advance!

 

[SnN Day :)]

Answering to myself, I solved the problem. I did following steps:

Firstly, I used fully white A4 (not required, but better if multiple) and other weight so light wouldn't get through
Secondly, used fully black A4 sheets and the same weight. Performed in a black room in evening
Thirdly, I matched the lines at the top of my big finger BUT the catch is that you need to press it quite hard through the screen AND use only the upper part of the finger (if you place your finger orienting on the circle on your finger it will raise one of 4 errors: 1515, 1507, 1503 and the last one about polar of you finger or something).