Active sync: can't find server

[Siegfried Van Eygen]

I installed Kolab 3.0 on CentOs 6.4.
I got everything working, except for Active sync.
When I log in <server-ip>/Microsoft-Server-ActiveSync I get 'It Works' 
Your ID= 2 Your Ip= <server-ip>. So I guess configuration is correct.
However, when I set up a connection via Outlook 2013 (inside the LAN), I 
got the message 'could not find Microsoft Active Sync server'.
I tried with the <server-IP> address, with the name of te server, no luck...
I can set-up a normal e-mail account, but of course, like this, I don't 
get calenders, contacts,...
I really don't know what is going wrong. Is it a Kolab problem? Is it 
network related?
How can I figure out what is going on, or even better, how can I solve 
this problem?

S. Van Eygen