Won't complete a scan: The preferences were updated by another process.

Hi all,  I am using Sophos Antivirus 7.3.10 on OS X Lion and every time I run a scan of the local drives it is halted after a short while with the message: "The preferences were updated by another process. Reloading the preferences from disk. "  The scan doesn't resume when I hit 'OK'.  This happens after only about 10% of the drive's files are scanned.  I have tried turning off all other programs when running the scan, and have set the computer to sleep as 'never', but its still happening.  Any ideas on how to fix this problem?

Thanks for your help,

cb