Not able to play some video files

Hi Guys,
I wonder if anyone can help

I recently bought a 4TB device for the purpose of watching video when away from home and all was fine for a few weeks and then suddenly it started offering to download some files instead of playing them!
The file types are in the list of playable extensions (.mp4, .avi and .mkv), sometimes if I refresh the page then they will play but not always, sometimes it will partially play a movie, then stop and offer download,

Thanks in advance

Here are some thoughts and a few things to check.

  1. Disable Plex if installed (assuming you’re using MCH mobile app for content streaming)
  2. Stop Windows / macOS backups if they are running
  3. Stop USB, Social Media imports etc…
  4. Disable Desktop Sync if enabled
  5. Reboot the MCH
  6. Stream content on the local LAN?
  7. Stream content on the carrier network?
  8. Stream movies on a remote WiFI network?
  9. Change resolution by switching from HD to SD in the app (if the option appears) when streaming

If content streaming is slow, buffer and resort to the download option, the problem will normally be internet upload speed or bandwidth limitation. IE: content streaming going outside of your local network is normally slower than incoming speeds.

I checked everything you mentioned if applicable (5,6,7,8), but unfortunately the same problem exists.
Internet speeds 81 download and 80 Mbps upload and it is higher on my home network, but I get no buffering or slow playing?