It seems like another process (probably Denodo 5.5 and Denodo 6.0) is running on the same port of the web container, or the port is not open.
You can open the Denodo 5.5 VDP admin tool and follow the below document to change the port numbers.
I have also found more information in the Advanced VQL Guide, in the section titled EMBEDDED WEB CONTAINER MANAGEMENT.
The commands "webcontainer start;" or "webcontainer status;" can be useful to you.
Surely this will help to overcome if both Denodo platform installations are trying to run on the same port number of the web container.
Hope this helps!