Single Sign on / Cross Domain Auto-Login not working as expected


I want to realize a "Single Sign On" with the Domain Mapping plugin.

My setup looks like this:

Domain #1: ""
Domain #2: ""

I mapped Domain #2 to "", which now is Domain #3.

Now I want to log in to Domain #1, then browse to Domain #3 and be already logged in. I activated the Cross Domain Auto-Login feature, but it doesn't seem to work.

What can I do?