This is commonly caused by a crash in the video driver while it is doing hardware-assisted transcoding. You may see a pop-up stating that video driver has stopped working and has recovered. To resolve the issue, please update the video drivers to the latest available version. 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.