I've resolved my problem: i've flashed TWRP 3.0.0-0 (cofface) using Flashify app instead of latest 3.0.0-r2 (flashed via fastboot before). So, to cofirm, i've reflashed 3.0.0-r2 with Flashify and reinstalled xiaomi.eu 6.3.17 ROM: BOOTLOOP again. Conclusion: there is something wrong with TWRP...