SBS 2003 to SBS 2011 Migration issues

Greetings,

I've been doing a migration of SBS 2003 to SBS 2011, and the
instalation process had no big issues.
I've followed the Microsoft recomendations an used the SBS MIgration
Assintant tool.
Apparently everything is working normally, but, I started to receive
complaints of some clients.

Both servers are connected to one network switch as well as the
clients working normally. The clients with problems are connected to
another network switch.
I'm using only one subnet and no VLAN's.

I've setup a static IP address in one of the clients with problems,
and I can contact it (ping, tracert) from both servers but the client
can't contact any of the servers.

There are no other servers in the subnet.
The only service that is completely migrated to SBS 2011 is the DHCP
Server, all the others still remain in SBS 2003.

Aparently the problem seems to be in the second network switch, but,
if I shut down the SBS 2011 everything works normally.

Has anyone encountered this problem?

I really need help.

Thanks in advance.

Best regards.