The dbFront Web Application is hosted by IIS as one of many web applications. By default the dbFront Web Application uses port 42057 to communicate with the dbFront Service.
This error means that the dbFront Web Application attempted to talk to the dbFront Service and received an invalid response.
The most likely cause is another service which uses the same port as requested by dbFront. You can easily instruct the dbFront Web Application and Service to use a different port.
To view which ports are in use you can run the following from an Administrator prompt:
Once you select another port then you will need to updated both portions of dbFront with the same port number. For instructions see:
NOTE: Don't Use Web Hosting Ports.
Ports 80 (HTTP) and 443 (SSL) are reserved for use by IIS (Microsoft Internet Information Server) and other web servers. Those ports are used when browsers connect to a web server.