@PabloW You must have entered a numeric PIN before exporting the backup, which means you need to use another program to extract the seed using your PIN
Through this tutorial I managed, following the tips of HCP, the guy who helped me, as well as @ HI-TEC99, in short, you have to download the program: decrypt bitcoinj seed >>>
https://bitcointalk.org/index.php?topic=1997031.0