A couple of things to try:
On the left side select existing service/group and select the one labeled "sqlnet" which should be 1521.
If that works, then it's a solution, although it doesn't explain why you are having the problem.
You might try in the "Create new member" area, in the text box put in "1521-1521" and see if it does the same thing.
I'm thinking that you may have encountered a bug in the ASDM software, so you might consider up/downgrading the ASDM install if this continues.
Sorry I don't have more to contribute, but it seems like you are doing everything correctly and if it's a bug there's not a lot you can do except pull a new version of ASDM down and replace the existing one to see if it fixes the issue.