Hello Simon,
according to the error message, the video within the file is not encoded in the expected format MJPEG. In such case it would be necessary to convert the file by using a third-party converter. Please note, MJPEG Is a simple encoding technique with each image frame being encoded as an individual JPEG and stored in sequence within the file. In Internet there are online converters available you can simply use via WebBrowser.
Does it help you further?
Best regards
Paul Banach