[AT&T LG G3] blueh25:
It worked !

Thank you for sharing this.

I have AT&T LG G3 D850.

I had similar issue. My phone had marshmallow installed on it, I tried to root it(using kingroot) but each time I reboot the phone the root access is lost. Then one time I was trying to edit some file after rooting the phone(to enable tethering, but eventually it didn't work for me) then I tried to unroot the phone manually by using kingroot before rebooting. The phone performed a reboot to apply the changes then it got stuck at the LG logo. I don't know exactly what went wrong. (maybe I accidentally messed up with some files but I don't remember anything like that).

I tried almost everything:

I started with factory reset(by using the power button and volume down trick), I confirmed the operation then I got an android logo window saying "Erasing" then the phone is stuck again at LG logo.

I tried to downgrade using lg flash tool 1.8.1.1023 to stock, but it didn't work. (I got an error before the progress even starts)

I tried LGUP (since I read somewhere that it is not possible to downgrade from marshmallow by using lg flash tool), then I got an error saying unknown model. (it seems like LGUP doesn't recognize my phone. All drivers were installed correctly because I tried another LG G3 D850 and it did recognize it)

I was trying to download LG pc suite (cuz I read somewhere that you can fix this LG logo issue by repairing the phone using LG PC suite), but the problem is that when I try to download LG PC suite I can't find LG G3 D850 in the list of devices.

Finally I found this thread and I have noticed that you have used different lg flash tool version(version 1.8.6.527) which seems to be newer than the ones posted by other members. Also I have noticed that the dll file is different (the one that you load along with the TOT file) . So without any hesitation I immediately installed that lg flash tool(after removing the old one), and went though the rest of the installation process. This time the installation progress at the lg flash tool started without any errors, the only issue here is I got factory reset status 2 instead of 3, but the rom was installed(the phone has been downgraded. Also now LGUP recognizes the phone, but I haven't used it fix this factory reset status 2 issue or re-installing a new rom).

Next I have removed this new version of lg flash tool and re-installed the old one back, then I loaded the stock rom again as usual(the dll one used here is the old one as well). Finally it worked without any issues and my phone is back to stock.

Some quick notes:

1-When using the new version of lg flash tool along with its dll, the model displayed in the tool is not D850 its something else (I don't remember exactly). But when using the old version of lg flash tool, it displays d850.

2-After reinstalling the old version of the lg flash tool at the end and trying to install the stock rom you might get an error that says ''can't change to download mode'', all you have to do is switch to another usb port or switch to another computer. Also the progress might take some time to start so give it 1 or 2 minutes.

I hope you can make use of this, especially if you have marshmallow in your phone and the phone is stuck at LG logo(or similar issue), and the LGUP tool doesn't recognize your phone.

This is my first smartphone (I have bought it few days ago) so I'm new to this and I don't know if there are better solutions, but this one worked for me and I hope you can find this useful to solve your problems as well in case you have similar issues. And of course the credits goes to oasevo who made this thread.