Am getting same message, ad provider could not confirm the video was watched. Tried several times, tried logging out and in again, clearing cache, with no effect.
1.122.20c56e28fe (12.03.2018 16:18), pengolodh-sc (137756), zz1, en_US, WIN 28,0,0,161, Windows 10, Chrome/65.0.3325.146, 1536x734, OpenGL (Baseline Extended) (336 MB VRAM)