Typically when the message "Backup is already in progress" appears it indicates that a failed Hot Backup has occurred.
What you will need to do is find the Medidata folder on the server/computer and delete the file HotBackup.txt. Usually, this is located at C:\Medidata.
This is a file that is created by Medisoft during the Hot Backup and is then removed once the Hot Backup is finished. If this file is still there then that indicates that the Hot Backup did not complete previously.