I changed the port # my Tridion CMS uses for the backend GUI then received the following error:

Unable to connect to remote server error

at System.Net.HttpWebRequest.GetRequestStream(TransportContext& context) at System.Net.HttpWebRequest.GetRequestStream() at System.ServiceModel.Channels.HttpOutput.WebRequestHttpOutput.GetOutputStream()

 

Tridion uses the WCF-based Core Service to retrieve items from the GUI. To solve the issue follow the steps in this post which describes how to change the WCF port number in the web.config