Old videos not playing.

I have a 6S Plus and for some reason the phone has stopped playing videos. I am not using a special app. These are videos that are in my camera roll that I recorded on the phone.

It will play newer videos, but if it's older than a couple of months I get a download error. I click on the play button and an exclamation point appears and when I click on that it says "Download Error" and nothing else. I hit "ok" to make the window disappear and that's it.

How do I get my old videos to play again??