It seems like mpv is trying to use swrast for a video driver, which I don't think will work with mp4. Check if libvdpau is installed (it should be as a dependency). As with your VLC problem @blaz, it seems like they're having trouble detecting the right driver. You can set it manually in ~/.config/mpv/mpv.conf with:

vo=vdpau
hwdec=vdpau

With nvidia I don't think you need the libvdpau-va-gl driver, but I don't have nvidia so I can't test. I'd suggest trying without adding any extra libraries first.

libvdpau is installed, I've created the conf file and added the content. Output:
blm@kolus ~ $ mpv /home/blm/Desktop/InShot_20171006_165639774.mp4
Playing: /home/blm/Desktop/InShot_20171006_165639774.mp4
(+) Video --vid=1 (*) (h264 720x720 29.855fps)
(+) Audio --aid=1 --alang=und (*) (aac 2ch 44100Hz)
[vo/vdpau] Error when calling vdp_device_create_x11: 1
Error opening/initializing the selected video_out (--vo) device.
Video: no video

Exiting... (Errors when loading file)

Installed libvdpau-va-gl, rebooted.

blm@kolus ~ $ mpv /home/blm/Desktop/InShot_20171006_165639774.mp4
Playing: /home/blm/Desktop/InShot_20171006_165639774.mp4
(+) Video --vid=1 (*) (h264 720x720 29.855fps)
(+) Audio --aid=1 --alang=und (*) (aac 2ch 44100Hz)
[vo/vdpau] Error when calling vdp_device_create_x11: 1
Error opening/initializing the selected video_out (--vo) device.
Video: no video

Exiting... (Errors when loading file)

This is really weird. Have you tried playing a different mp4 file? What happens if you try to play something like a .webm or other type of video file?

@blaz any update on this ? it's been 3 months since your last comment.