[G3 Q&A, Help & Troubleshooting] DrFlaisch:
EDIT: Some kind of voodoo happened right now! I was about to unbrick again and wanted to give the phone a try to boot before. And it did! So no problem at the moment, but still a strong feeling of uncertainty...

EDIT: Phone is still randomly rebooting and gets stuck in bootloops :(

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Hej guys, I really need your advice,

but first things first: Thank you very much for all of your wonderful work on custom roms and further support of older devices. I've been reading here for a few years now and was more or less happy to have up-to-date-roms for some of my phones.

Now I'm facing quite the same problem like OP did / does. Everything started with me, following this guide to root my D855 (coming from stock rom) and flash TWRP: you tube.com/watch?v=8VOgM3jLas4 (Note: The guide told me to flash "3__HacerPermisivo.zip" for updating the kernel, which I did.)

Next, I flashed a newer version of TWRP ("twrp-3.2.3-0-d855.img") than the one provided in "2__AutoRecMMD855.apk" by the guide above. I wiped dalvik and cache and after that, I flashed "lineage-15.1-20190410-nightly-d855-signed.zip", wiped dalvik and cache again and flashed "open_gapps-arm-8.1-pico-20190411.zip". I wiped dalvik and cache again and told TWRP to reboot.

Here is, where problems began to occur. The phone got into a bootloop, showing up the LG splash screen again and again instead of booting into LOS correctly and set it up. At some point I decided to remove the battery. When I did, at the same time LOS was just going to load, which I saw because of its beginning boot animation. So I screwed it up right at the wrong moment, I guess.

From here, the phone bootlooped every time, I wanted it to start. No battery acrobatics helped and on top I couldn't get it into recovery mode. Any attempts ended in bootloops. I think, this might be what is called a brick / to brick your device, isn't it? I found this guide to unbrick a D855 and followed its steps: open -freax.fr/guide-unbrick-your-lg-g3/

Believe me, it was a real pain in the ass! Because of me, messing up something or the phone getting stuck into a bootloop during the unbrick process, I had to do it 4 to 5 times until success... I have to say: I didn't follow the last steps beginning from "Root". Instead, I followed the guide from the beginning of my post.

Finally, I got "lineage-15.1-20190410-nightly-d855-signed.zip" and "open_gapps-arm-8.1-pico-20190411.zip" to run. I set it up and began to load some of my apps of my Play Store collection. While downloading, the phone froze, crashed and began to bootloop, from here unable to boot normally or to get into recovery mode. So I unbricked and flashed again (again several times because of bootloops before LOS first set up). When I got it to work again, I was able to install my apps without being trolled by the phone again. But scrolling through the eBay app froze the phone and it began bootlooping again...

You can believe me, I was about to throw the D855 against the hardest piece of wall, I could possibly find in my rage. The reason why I didn't give up at this point is, that I LOS-rommed two other D855s just a few days before. Those were my mom's and my girlfriend's and they work fine until today. I bought this third one for myself, because I was impressed, how good LOS worked on the D855s. On top I wanted to get rid of my Samsung Galaxy S4 (GT-I9515), because LOS kinda sucks on it, but this is another story...

So I figuered that the earlier LOS version I used for the women's phones, could solve the problem. So I unbricked again and flashed "lineage-15.1-20190327-nightly-d855-signed.zip" and "open_gapps-arm-8.1-pico-20190329.zip". And hell yeah, that did work! No bootloops, eBay worked fine, just everything was as I loved it on the other phones, I flashed. Until tonight. I've been lucky for almost two days, but tonight this happened: I plotted a route in Google Maps and saved it as a shortcut on my homescreen. I closed Maps and tapped the route icon. Freeze - crash - bootloop - no recovery mode! I wanted to start crying, because Maps has already worked fine yesterday. And it still does on the other two phones, which run exactly the same rom...

So now, I'm sitting here, not knowing what can be done further. I'm not a programmer / developper or any of those cracks providing us the newest software for old phones. So I just can do wild guesses. Maybe the moment I removed the battery the very first time after flashing messed something up very deep inside the phones internals and it can not be cured any more. Maybe some of you guys have experienced similar things and know a tweak or a simple step, I forgot. Anyway, I hope I can use my phone on LOS some day which I bought especially for this case.

Keep up the great work, folks!

Greets