LINE Letter Sealing - Unable to display message as it couldn't be decrypted
Had the same problem. The second method on this page worked for me.

Steps:


Do a previous backup
copy /data/data/jp.naver.line.android/databases/naver_line to your PC and rename it to naver_line_old
Reinstall and do a fresh setup of LINE
copy the new naver_line to your PC and rename it to naver_line_new
In your PC, open and copy the chat_history table from naver_line_old(I used SQLiteStudio) and paste it on naver_line_new, having previously deleted chat_history from it
copy naver_line_new to /data/data/jp.naver.line.android/databases/naver_line(rename it as naver_line)
Restart your phone


Letter sealing should work OK now; unfortunately, I don't know how to make the problematic messages readable again.

I hope to have solved your problem.