Fabrice,
let me first recommend you to upgrade to a latest 7.40 version. The one you currently use is the first one of this train and many many fixes have been introduced since.
Regarding your problem, are you saying that each time you want to access server-1 you get to server-2 and vice versa ?
Is it all the time ?
If you're going to a server address directly, the CSS does not do anything to the traffic. It just routes it.
Did you capture a sniffer trace in front and back of the CSS to see if the CSS modify anything ?
Could you send us your CSS config ?
Did you verify mac address on the CSS for the server ip ?
Regards,
Gilles.