Incoming communication (to Connect)
Make sure that any existing firewalls allow access to the desired resource.
Outgoing communication (from Connect)
Make sure that any existing firewalls allow access to the desired resource.
If a browser is installed on the Connect server and the request is an http GET request, try to access the resource with it. Otherwise use Invoke-WebRequest (Windows PowerShell) or curl (Linux / macOS) to execute the request. Check the corresponding error messages.
On the Connect server, create a connection to the desired target system and run a connection test (if available). If necessary, check the error messages that appear in the user interface and on the console.
Advanced error analysis capabilities
By setting the "EnableHttpRequestLogging" option in the "Debug" section of the Connect Server configuration to "true", all incoming HTTP requests are logged on the console. This can be used to check what exactly is received from the Connect Server.