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.