I had a few users over the past week(s) report that they could not connect to the local Team Foundation Server.  The message received was  “TF31002: Unable to connect to this Team Foundation Server”

Each attempt to connect returned the same results.  Using Internet Explorer they were able to browse to http://<server-address>:8080/tfs however Visual Studios was unable to connect.

Reviewing the system proxy configuration shown that there a proxy configuration was not in use; or was it.  This is where I started to look for their party software which could pose such issues.

And then the problem was solved. The issues was caused by malware / adware being installed on the users system.  Once removed all was normal again.

More on this here