This only happens when successfully opening a media source AFTER failing to open another media source.
1) Open a MediaPlayer asset
2) In the media library tab, try to open an invalid media source
3) the URL box above the media player viewport should now be red
4) Now open a valid media source
Result: The media source opened successfully, but the URL box is still red
Expected: The URL box should be white, indicating success
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-37470 in the post.