There is a know issue in CrashPlan that causes backup to fail if Sophos finds a false positive issue with the cache directory. The resolution to this issue is to whitelist the cache directory from Sophos scans.
The procedure to perform this is as follows:
Open the Sophos menu through the icon in the upper right corner of your desktop, and select Open Preferences.
In the Preferences window select the On Access panel. Select Excluded Items, click the + button, and enter /Library/Caches/CrashPlan. You may then close the preferences window. Note: to perform these steps you may need to unlock the application using the lock icon in the lower left corner.
