dialog instance vs central instance

Hi,

I've a central instance (r3 640) and when install an additional dialog instance (first) in a different machine the communication between both (into sap) require logon, i.e. from CI in Tx sm51 when doble click to DI appear the logon screen.

Any ideas of what's wrong???

The services in both machines are equals and no errors appear in log.

Thanks

Jose