"System.Security.SecurityException: Security error" error message when the virtual directory points to a remote share in ASP.NET

To resolve this behavior, grant the FullTrust right to the remote share:

1. On the Web server, open Administrative Tools, and then double-click Microsoft .NET Framework Configuration.
2. Expand Runtime Security Policy, expand Machine, and then expand Code Groups.
3. Right-click All_Code, and then click New.
4. Select Create a new code group. Give your code group a relevant name, such as the name of the applications share. Click Next.
5. In the Choose the condition type for this code group list, select URL.
6. In the URL box, type the path of the share in the following format:
file:////\\computername\sharename\*
Note Replace computername with the name of the computer that is hosting the remote share. Replace sharename with the name of the share.
7. Click Next. On the next page, select Use an existing permission set, and then select FullTrust.
8. Click Next, and then click Finish.
9. Restart Microsoft Internet Information Services (IIS) to restart the ASP.NET worker process.

If Microsoft .NET Framework Configuration is not displayed under Administrative Tools, you can install the .NET Framework SDK to add Microsoft .NET Framework Configuration. Alternatively, you can run the following command to make the change:
Drive:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\caspol.exe -m -ag 1 -url "file:////\\computername\sharename\*" FullTrust -exclusive on
Note For more information about what these arguments do, run the following command:
caspol.exe -?

Oh, and don't forget to use:

aspnet_regiis.exe –ga UserAccount