Robododo ok, that then should not be the issue.
Did you download the videos with the phone?
I downloaded videos from the website and put them onto a new micro sd card in my Galaxy S7
I took this as meaning, downloaded to PC and transferred them to SD. It's not uncommon for large media file downloads to a mobile device/SD card to have incomplete or corrupted files. A download manager in every instance I've come across from users fixed the issue.
Another thing I just now think off, are the video files h264 and larger than 4K?