I have the same issue as above. Video files will not open in either gallery or file manager on my Galaxy S7 android 8.0. Gallery will show a video preview which can be viewed with sound but selecting the video will give the msg "file not found". Opening a video file from file manager will cause file manager to crash (no video will open). If you attempt to open a photo from file manager it will give a msg along the lines of "gallery has crashed". If you try to open a photo from gallery it opens just fine.
All this has been done after clearing the system cache partition, affects both updated and downgraded versions of gallery and leads me to believe it may be a programming issue withing android 8.0.
*Side note- after adding an mp3 file to my typical music folder, the default music app did not recognize that the file existed. I attempted a restart but it still did not recognize that there was a new file. I gave up and let it sit overnight and now the file is in my library. It is odd because music usually shows up immediately after it is added.