Do you use an older version of Debian?
+1?
I was using an older version of Debian. Now I have V9.1.0 and it is working.... However, now I can get a readable "wallet.txt" at step 5, but in plaintext it says "Seed is encrypted" and although it shows Xpub (for a watching only wallet) all of the private keys are shown as:-
"encryptedPrivateKey=EncryptedData [initialisationVector=[104, -105, -15, 72, -51,.... etc".
Presumably there needs to be additional calculations done to extract the actual private keys?
Did you follow step 3 to decrypt the wallet? You will be asked for the password of the backup.