Visual Studio - error on running performance test

I created a performance test using Visual Studio 2012 Ultimate. When I click to run the test, I got the following error message:

The following agent cannot be used to run tests because it is offline: ; Failed to queue tests for test run on agent : Object cannot be stored in an array of this type; No agents are marked online to execute test run.

The strange thing is that it happens to newly created performance tests / load tests only. The performance tests / load tests I created months ago runs perfectly fine. I found this problem when adding new performance tests. Anybody any ideas? Researched but didn't find the solution. Thank you!