For those of you interested:

If you upgrade your ISC/AC, it is important to remember the username/password 
with which the ISC/AC was installed with.  If you forget your username/password 
combo [usually (iscadmin/iscpass)], you could have potential problems of not 
successfully being to upgrade.

If by some chance you change your password combination, there is a proper way 
to do this:
[IC55394: FULL STEPS FOR CHANGING THE ISC INSTALLATION ID PASSWORD ARE NOTCLEAR 
AND CAN CAUSE ISC/ADMIN CENTER UPGRADE FAILURES]

http://www-01.ibm.com/support/docview.wss?uid=swg1IC55394

In the end my upgrade from AC 5.4.0.0 (ISC already at 6.0.1.1) was unsuccessful.

The end solution was to uninstall the ISC and perform a fresh install of the 
ISC and the AC.

In case you wanted to know.