Symptom
The Designer cannot connect to a Runtime.
The Runtime is up and can execute deliveries.
The Runtime server can ping the Designer's machine, and vice versa.
Solution
Edit the file stambiaRuntime/properties/engines/commonParameters.xml on the Runtime server.
Change the following line:
<!--<parameter name="rmiHost" value="localhost"/>-->
to this:
<parameter name="rmiHost" value="<hostname of the Runtime server>"/>
And then restart the Runtime.
If it does not work, try with the IP address instead of the hostname :
<parameter name="rmiHost" value="<IP address>"/>