Seems like you forgot to cherry pick this commit from backup_restore.rb into 1.6.2 / 1.6.3 so the constant is missing…
Resulting in:
Completed 500 Internal Server Error in 58ms (ActiveRecord: 34.9ms)
NameError (uninitialized constant BackupRestore::OLD_DUMP_FILE)
/var/www/discourse/lib/backup_restore/restorer.rb:118:in `initialize_state'