full system scan keeps getting error last 2 scheduled scans

I do a scheduled full system scan every Saturday. For the last 2 weeks iIget an error message and the scan can't be completed. After rebooting my computer, I try to do a manual full system scan and it works. Why isn't a scheduled full system scan no longer working?

Holly

Ps I have a feeling it's relate to an update as tis has never happened before.