File permissions issue
This problem occurs when the NAS device cannot provide Symantec AntiVirus Scan Engine with access to the file. To resolve the problem, ensure that the Symantec AntiVirus Scan Engine service has full permissions to all of the files that are on the NAS device.

If Symantec AntiVirus Scan Engine is denied access to the file for scanning, the filer prevents client access to the file and reports the error.
You may also decide to permit access to specific file extensions on the NAS device without Symantec AntiVirus Scan Engine scanning.

Corrupt file issue
If the problem is not due to permissions it is possible that the file could be corrupt. This file should be removed from the Filer and further examined to confirm that it is a corrupt file.

Scan Engine unable to scan the file issue
Allow access to these files using the one-liner
java -jar xmlmodifier.jar -s //protocol/AllowAccessOnScanError/@value true configuration.xml

Central Quarantine is full
Make sure the Central Quarantine has ample room for submissions from Symantec Scan Engine

Technical Information
Because the problem occurs prior to the file scan, Symantec AntiVirus Scan Engine does not make a log entry for the error.