Symptoms
When trying to encrypt/decrypt the web.config file, an error similar to the one below is displayed:
Unable to save config to file 'C:\.......\web.config'.
Cause
The permissions of the .NET user account are not set right to allow access.
Solution
The .NET user account (typically ASPNET for WindowsXP or NETWORK SERVICE for Windows Server 2003/Vista or IIS_IUSRS for Windows Server 2008/Windows7) must be given read/write/modify access to the folder the web.config file resides in. This is typically the {root} folder. Remember to reset the permissions once the web.config file has been encrypted.