Not sure if this is going to be useful to anyone but I lost quite a lot of time figuring this out, so I wanted to share it:
I was having trouble updating from Lineage 16 to 17.1 (using this manual https://wiki.lineageos.org/devices/klte/upgrade), getting bootloops every single time. I tried various 16 and 17 roms and in the end found ONE rom that didn't go into a bootloop but gave me a proper loading screen (a 2019 Lineage 16 build).
VERY long story short, with exception of the 2019 Lineage 16 rom every single rom I tried went into a boot loop until I unplugged the USB cable. If the USB cable was connected when the boot sequence started they all kept rebooting either while showing the boot animation or after flashing the first setup screen.
I don't know if that's a common problem but if it is I think it should get added to this page: https://wiki.lineageos.org/devices/klte/upgrade
edit: I'm getting inconsistent results now, but I have a video in which it was consistently looping with the cable and booting without
edit 2: Aaaand I'm back in bootloop hell. I did manage to flash the 2019 Lineage 16 ROM and boot into it, but even that is working inconsistently now. I've tried 16, 17.1 and 18.1 at this point.
My working 17.1 installation went to crap when I flashed Magisk. I'm not sure if that was actually the culprit though. This is by far the fussiest upgrade I've ever had to do.
edit 3: I've since cleared everything, installed the 18.1 recovery, tried to flash various images multiple times and after a long time I managed to boot into a clean version of 17.1. It seemed fully functional... until I rebooted it.
It is now in a bootloop again.
I don't know how to troubleshoot something that works once in a long while and reboots at different times in the boot process as well.
It booted normally, I restarted it and this happened:
1 long buzz and reboot after two logo animations
2 long buzz and reboot after two animations
3 short buzz and reboot after two animations
4 no buzz except for the "normal" boot buzz, reboot after two animations
5 no buzz except for the "normal" boot buzz, reboot after two animations
6 no buzz except for the "normal" boot buzz, reboot after two animations
7 no buzz except for the "normal" boot buzz, reboot after two animations
8 long buzz and reboot after two animations
9 long buzz and reboot after two animations
10 double buzz after two logo animations and regular boot into 17.1
The phone was working fine on Lineage 16... and now I can't even consistently boot into that.
edit 4: Flashed lineage-16.020190407-nightly-klte-signed and it's booting consistently again.
edit 5: Flashed lineage -17.1-20201227-nightly-klte-signed, it booted into android, I was already going through the menu's and it rebooted. It's in a bootloop again.
edit 6: I had about four succesful manual reboots on 17.1 and now it's bootlooping again (3 times so far)
I can't get a consistent result. Does anyone have any idea what I could try?
I get same problem with L17.1 , i am using scl23(kltekdi)... but L16 last build is working for my device. I dont know whats the problem here .. I did clean flashing and also tried installing update via Lineage recovery but i get nothing but endless bootloop. Btw, thanks for this update.