I had the same problem with UE4.25.
To solve this issue, I have realized that it’s because I was putting unreal files in the same folder than my image sequence.
So create a sub-folders for your image sequence (one folder by sequence ie (/movies/sequence1) and save the image media source in the root folder (/movies/).
Same thing when you create your Media Texture and Media Player. Put them outside of the folders of your image sequences.