Recently we got the below error while trying to remove the marketing list member from a marketing list.
This occurs if the marketing list is locked. The marketing list gets locked if it is Dynamic Marketing List. However, we are getting this issue on our Static marketing lists. Later we realized that those static marketing lists were created using Copy To Static ribbon button. When a Dynamic Marketing List is created the locked field value is false. However it gets set as true when we convert it to Static using Copy To Static functionality.
Changing the Locked field value as No fixed the issue.
Hope this helps..