Backup file invalid: fix broken Threema backup
Extract backup and ignore Unexpected end of archive error…
7z x threema-backup_*.zip
… and recompress with password.
cd threema-backup_*
7z a -p -mem=AES256 -tzip ../threema-backup_<original_filename>.zip *
Check compression method and encryption (Method = AES-256 Deflate):
7z l -slt threema-backup_<original_filename>.zip