Here's what worked for me:
-Like all of you I would get the secure boot error. To fix it I decided to reinstall the software from the official LG app on PC (Windows) and it worked!
Official LG app on PC
The ability to enter download mode on your phone
A cable to connect the phone to the PC
How to do it:
-Turn of the phone completely, the enter download mode by holding the vol+ button and the inserting the USB cable before it reaches the LG logo (it's precise timing so it may take a few tries). If you did everything right you should get a message saying entering dowload mode and see a screen in which there is a USB logo and progress bar and some other writing. On your PC go to the LG app and it will offer you the latest software update. Do the update. It will download the software the upload it to your phone and after installation it will power on and voila your phone is alive and there is no data loss at all
-I hope this works for all who have this problem and since this is my first reply/post ever I hope it's useful.