Underlying connection was closed
When users save work items, check in code merges, or perform other Team Foundation operations on a large number of files, a dialog box might appear that contains the phrase "The underlying connection was closed." The problem might be intermittent and difficult to reproduce.
The client computer might not be configured to automatically detect the link speed of its network adapter. This setting can decrease performance between the client computer and Visual Studio Team Foundation Server.
On the client computer, enable automatic detection of the link speed for the network adapter. For more information, see Configure a network adapter to automatically adjust speed.