False positive on Ghost 15?

My installation of Norton Ghost 15 is set to do a file/folder backup daily and a C drive backup once a week, with a new recovery set once a month.

 

Lately, it has begun to issue the following error message following the C drive backup (the f/f backups do not exhibit this behaviour):

 

"Attention needed : the last scheduled backup of OS (C:\) was missed

 

Remedy: back up now to create recovery point"

 

Checking the status shows a tick, and in the destination folder, a backup file with the correct date and time is present. What's going on here?