So after messing around with this in various ways I found that if I change the access fields to be some nonsense values (but don't actually try to connect) it resets the underlying data so that I can then re-enter the real data (ssn/uname/pwd) and continue. This worked and I was able to download.
Full disclosure I did save and exit TTAX - (which did not appear to help), but it was part of the steps I took.
This is certainly a bug in the U/I based on the principle of least surprise. It may be an attempt to prevent a user from locking themselves out, but it should tell you that.
Thanks