To me it looks as though the error comes from a missing value in the logon details for the FTP server. Have you verified that both username and password were entered correctly?
As for upgrading, I would upgrade the ASA first to version 9.1(3) and then upgrade the SSP-20 module. When it mentions upgrade to 9.1(1) and then to 9.2(1), this is just a minimum version to be able to upgrade to the next level. So if you upgrade to the highest build within a minor version, it will support upgrade to the next minor version.
You still need to follow the upgrade path but as mentioned, it is a minimum requirement to be able to upgrade to the next step.
Tested upgrade paths are for the most recent build number for a given version, and sometimes the second
most recent build. For example, upgrade from 9.1(1) to 9.1(3) is tested with build 9.1(1) Build 21; 9.1(2)
to 9.1(3) is tested with 9.1(2) Builds 29 and 42. To ensure the best results, you should upgrade to the
most recent build for the release you are running before upgrading to a new version.
Thanks again. I was asuming the error was related with the update but in fact it was that the ASA module did not like to connect to a ftp server with a user and blank password. Giving a password to the ftp user resolved that.
Finally i got the desired update. This is the path i followed:
update ASA version 8.4(6)
(with ASA on 8.4(6)) update CX from 9.0.2 (103) To 9.1.2 (29)
(with ASA on 8.4(6)) update CX from 9.1.2 (29) To 184.108.40.206 (48)