As a test this morning:
I have a random Cas3+ encrypted; no Key and no ECU. I did firmware downgrade and went to key learning. Selected option 4 (all keys lost), entered '00000000' when asked for DME SK, loaded a prepared file for my test key, inserted key when asked. It programmed my test key to the CAS perfectly; I grabbed a random ECU (that uses 4 byte ISN), aligned it to CAS and all work perfect. Took 15mins (including cas downgrade).
So not sure why the guys above are having problems? I must admit earlier the week I had an issue, but it turned out the key ribbon I used on my test bench was worn from repeated use. Fixed ribbon and all worked perfectly.