SBS 2003 to SBS 2011 migration problems

Greetings,

I'm doing a migration of a SBS 2003 to SBS 2011 and I've followed all
the steps recomended by Microsoft.
The instalation process was quite straightforward using the migration
assistant software, but now i'm facing a very strange problem.

At this point I have both servers online. The SBS 2003 still has most
of the roles except DHCP Server.
My infrastructure has only 2 network switches.
The clients connected to the same switch of the servers are working
normally.
The clients that are connected to the other switch, can't get an IP
address from the DHCP Server, and, if I setup a static IP address I
can contact them from the server (ping, tracert, ...) but the clients
can't contact the server.
I only have one subnet, no VLAN's and there are no firewalls in
between.
There are no other servers in the network.

The strangest thing is, if I shutdown the SBS 2011, everything works
normally.

Has anyone been though something similar?

I really need help.

Thanks in advance.