there has to be at least two people in the meeting. If there is only 1 person in the meeting, it is not considered a meeting and the API will return the “Meeting ID is invalid or not end.” error, or the empty participants array.
For the meetingUUID you tested with (Wd5AX70ZROSniZxBI3J7/A==) you (the host) were the only one in the meeting, so the participants array was empty since there were indeed no participants.
Try starting a meeting and having two people join (Yourself and a participant) and the endpoints will return the expected data. (Try passing in the meetingID and then the meetingUUID. If the meetingUUID contains a / you will need to double URL encode it.).