In Visual Studio Team Lab Management, the following error message is observed on a testing capability enabled lab system when the lab system had lost trust relationship with the domain.
TF267055: The machine is not ready to run tests because of the following error: Unable to connect to the controller on 'aseemb-tc.fareast.corp.microsoft.com:6901'. The agent can connect to the controller but the controller cannot connect to the agent because of the following reason: Authentication failed on the remote side (the stream might still be available for additional authentication attempts). No authority could be contacted for authentication.
To confirm that this is indeed the case, do the following.
1. Log-in to the test controller machine.
2. Try to open a file share for the lab system from the test controller machine. (If trust relationship is the problem, then you will not be able to open the file share and it will fail with trust relationship error)
To come out of this error, rejoin (unjoin first and join it back) the lab system back to the domain.