Guess it's worth to skip-read back in forum posts once in a while as fast forward to June 2022, this did it for me, so you shall be my saviour!
After spending about two days with this piece of crap software what Line technically is (unfortunately extremely widespread and popular in Asia and especially in Japan) with force close here and splash screen doodling there despite copying myself clumsy and stiching obscure SQL-databases together, finally the braindead patient could be sucessfully reanimated.
Uargh, I really wonder what kind of retarded developers at "Naver" are responsible for this mess. Not only are they apparently still incapable of providing a decent backup function (which one would actually expect to be provided system-wide by Android itself to be fair, however Google isn't much better here), but they also make it especially difficult and painful to backup anything beyond a few recent chats in the first place. That in conjunction with tenshousands of distributed files in even more subfolders after some years of usage which one can manually copy and shift around, totally insane.
I wish the Line developer team to lose their phones, all their content while being in urgent need to lookup some chat or images from the past. Maybe then they may get a glimse of how ****ty and moronic it is.
Alas, they might be using the iPhone version of the app. My iPhone using friends mentioned they can backup everything, photos and all. Not sure if it is because the iOS do backup better (TBF, I think they do, LINE isn't the only app where Google's backup is half-assed), or Naver did a better job with the iOS version.
One thing I wonder is how are chat saved on their server and for how long. I noticed some "interesting" behaviour recently. Basically, if you manage to restore LINE on a new phone, it is sort of possible to run LINE on two devices. LINE sometime detects it and will try to wipe one of the phone, but you can usually force close and void that from happening. You can also avoid this from happening by not having both phone switched on at the same time. Any recent messages will also be received on the backup phone once you switch it on. I used to take that as evidence that LINE basically store everything anyway, but it seems like that is not the case.
Recently my main phone suddenly died without warning. At that point in time I hadn't switched on my backup phone for a while, and left it in a different city. Several months later, when I finally got hold of my backup phone and switched it on, LINE was still working, but 1. Messages from my main phone received after I switched off the backup phone are forever lost, LINE did not bother pushing those messages to the old devices. However, those messages can still be viewed on the PC version which I used in conjunction of the main phone when it was still working. 2. A number of messages sent between the time my main phone died and switched on my backup phone were also forever lost somewhere. LINE won't deliver messages that are several months old if if you don't log into LINE either via phone or PC within a time period (if your device is off for a few days that is fine, but a few months is NG - I am not sure what the cut off time is). This behaviour is quite different from other chat I have used.
What this means is that while I can keep LINE installed on older devices to keep as backup in case something happens to my main device, if I don't periodically switch it on to allow the backup devices to receive recent messages within a timeframe, the backup will be incomplete.
(Not sure if this information will be of any use to anyone, but lesson here is to either periodically switch on the backup device or regularly backup your LINE if you don't want to lose anything if something happens)