[Samba] big problem with cifs

[antonello]

Using a Zentyal 3.0.2 server with samba4.
I have big issues connecting linux clients to the server (we have a few 
windows clients and they work).
I connect the home directory via samba.
Homes are stored in /home/likewuse-open/MYDOMAIN/
But it works erratically, most of the times users can't connect and 
client freezes.
Logs show me that is must be a question of non connecting to the server:

cifs vfs: send error in SETFSUnixInfo = -11
server non responding for more than 120 seconds...

Yet, the network works (as the windows clients show). I tried to attach 
the server and only one client to a separate hub and still I got the 
same problem.

what shall I do to find out where the bug is?

TIA
Antonello