Microsoft Forefront Server Security encountered an error while performing a scan engine update.
   Scan Engine: Norman
   Update Path: http://forefrontdl.microsoft.com/server/scanengineupdate/x86/Norman
   Proxy Settings: Enabled
   Error Code: 0×80004005
   Description: An error occurred while checking if an update was available.

Microsoft Forefront Server Security encountered an error while performing a scan engine update.
   Scan Engine: Norman
   Error Code: 0×80070102
   Description: Unable to acquire the scan engine update mutex within the designated timeout period.

And in ProgramLog.txt

"INFORMATION: Attempting to download the Norman scan engine package from http://forefrontdl.microsoft.com/server/scanengineupdate/x86/Norman."
"INFORMATION: The proxy settings will be used for this connection."
"ERROR: The Norman scan engine update timed out while downloading files"
"ERROR: UpdateException: GetFileCommand failed on norman_fullpkg.cab.  (0×00002efe) The connection with the server was terminated abnormally.  WinHttpClient failed while reading data."
"INFORMATION: The Norman scan engine has been rolled back."

"INFORMATION: Attempting to download the Norman scan engine package from http://forefrontdl.microsoft.com/server/scanengineupdate/x86/Norman."
"INFORMATION: The proxy settings will be used for this connection."
"ERROR: Unable to acquire the scan engine update mutex within the designated timeout period (300 seconds)."
"ERROR: The Norman scan engine update timed out while downloading files"
"ERROR: UpdateException: GetFileCommand failed on norman_fullpkg.cab.  (0×00002efe) The connection with the server was terminated abnormally.  WinHttpClient failed while reading data."
"INFORMATION: The Norman scan engine has been rolled back."

"INFORMATION: Attempting to download the AhnLab scan engine package from http://forefrontdl.microsoft.com/server/scanengineupdate/x86/AhnLab." 
"INFORMATION: The proxy settings will be used for this connection." 
"ERROR: Unable to load manifest from: http://forefrontdl.microsoft.com/server/scanengineupdate/x86/AhnLab/Package/manifest.cab : WinHttpClient send request returned an invalid return code 404."

This happens because timeout occurs while updating the engines. The solution is to increase the default 300 seconds timeout period. This can be done by editing the following registry key:

Notice that you need to edit a registry value, so make sure you take a backup of your registry before changing any value.

Locate the REG_DWORD registry value EngineDownloadTimeout in the key:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Forefront Server Security\Exchange Server

And increase it to 600 seconds and try to update, if the update fails again then increase the period. If it is not found, create it.

Another important thing, make sure that the engines updates schedules don’t overlap.

I found that the solution was to install Hotfix Rollup 2 for Forefront Security for Exchange Server Service Pack 1 (KB 946864).