This is commonly caused by a crash in the video driver while it is doing hardware-assisted transcoding (you may see a popup stating that video driver has stopped working and has recovered, this is also an indication). To resolve the issue, please update the video drivers to the latest available. Note that if you have a system with a GPU integrated into the CPU and are using switchable graphics (Virtu, Optimus etc.) with a dedicated GPU, you may need to update both the integrated video drivers and the dedicated video drivers.
PCMark 7 v1.4.0 update includes fixes to the video transcoding test that should resolve any other issues you may have with this test.