This error occurs if some background application interferes with 3DMark and tries to draw a popup window, an overlay or otherwise switches the window focus from full screen 3DMark view during a benchmark run. In such cases the score would no longer be accurate so the test is aborted.
Most common culprits causing this error include various instant messaging and email applications that give popups when a new message arrives. Turn off such applications and try again. In general it is recommended to close any unneeded background applications before benchmarking a system to ensure that all system resources are available during the test.