LINE Letter Sealing - Unable to display message as it couldn't be decrypted

LINE is a chat application. I recently reinstalled my OS, and restored my LINE chat history using this procedure . Problem: Some of my chats appear like this: It says: > Letter Sealing > Unable to display message as it couldn't be decrypted. Please ask your friend to resend the message. As you can see, my friend re-sending me the message does not solve the problem. On the messages from July 6 to the day I reinstalled have this problem. So I disabled Letter Sealing, and now I can read new messages sent to me (but I can still not read the existing problematic messages). Questions: - How to make the problematic messages readable again? I still have the Titanium Backup from the former OS, if necessary. - Is there a way to re-enable Letter Sealing while making sure that encrypted messages will stay readable even after reinstalling my OS in the future?