Warning message in Daily Backup Log

I have Acronis TI Home (2010 version) with Windows 7. I have been running my daily backups successfully for some months now, but suddenly I am getting the following warnings in my daily backup log:-

"File I/O Error", and then "The system cannot find the file specified" After this I receive "Operation has succeeded".

As it seems to be completing OK eventually, I probably don't need to be too worried, but wondered whether anyone might have an idea to avoid the two warnings a shown above? Many thanks.