Domain mapping and I'm halted

Hi guys, if you can pls. help me with my situation, maybe I’m just overseeing what’s apparent…
I have two subdomains a.domain.com and b.domain.com, which I need to have networked.
I can not install WP to the root domain.com as a different CMS runs there.

So I decided to have a.domain.com as a primary domain and set up a network in there. So far so good.
Secondly, I set up b.a.domain.com in the network and need to have it mapped to b.domain.com as it should be.

I installed WordPress MU Domain Mapping plugin and activated it in the network. Now when I go to Superadmin – Settings – Domain Mapping and try to fill-in the cname for the server (a.domain.com), nothing happens and the page just refreshes without anything changing.

What am I doing wrong for goodness sake and what should be following steps thereafter?

– both a.domain.com and b.domain.com run on the same server
– I use different DB prefixes for the sites, i.e. a_ and b_ instead of common wp_ for both of them
– there are WP 3.3.1 installed on both sites
– plugins are disabled for now
– I use same admin for both sites

Many thanks for your kind advice.