Just checking as I also recently had issues...
I am using 180 degree too and the background video works normally. However, if I shut down my PC and restart, I get the same message "Play background video adbormally".
I took out the SD card and found that the video file "filename.mp4_180.mp4" is corrupted.
After deleting the file, I used the software to run the display. It will upload a new copy of the background into the screen/SD card and all is working fine.
But, after shutting down the PC, somehow the video file gets corrupted again. I tried by mounting the SD card and playing the video files directly, it will not play at all. I noticed, only the background video file that is being used by the theme before shutting down gets corrupted. Other video files that I upload for screen start up doesn't get corrupted.
I tried to format the SD card and have the software upload the video files into the SD card, but somehow, the background video file still gets corrupted after shutdown.
This happened recently. Had been running the screen for a year and only recently did this start happening.