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

Hello,

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

My setup looks like this:

Domain #1: "sub1.sub2.example.com"
Domain #2: "sub3.sub1.sub2.example.com"

I mapped Domain #2 to "sub4.sub2.example.com", 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?

Best
Stefan