This error will occur if there is a custom process that gets kicked off in the background to lock the attributes on the status report. There is a delay between the UI showing the attributes as locked and the attributes being locked on the back end. A change to a field is not allowed once the attributes become locked, so this is the reason for the error. The attributes will show as locked once the status report properties page is manually refreshed. They will appear as locked once the error comes up as well.