When navigating to the Web Interface website.

Internal Error
The Web site is experiencing technical difficulties. We apologize for any inconvenience.
The internal error may only be temporary. Try reconnecting and, if the problem persists, contact your system administrator.

A more detailed message is available after disabling the default error message:

Server Error in '/Citrix/AccessPlatform' Application.
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The current identity (NT AUTHORITY\NETWORK SERVICE) does not have write access to 'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files'. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: The current identity (NT AUTHORITY\NETWORK SERVICE) does not have write access to 'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files'.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace: 

[HttpException (0x80004005): The current identity (NT AUTHORITY\NETWORK SERVICE) does not have write access to 'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files'.]
   System.Web.HttpRuntime.SetUpCodegenDirectory(CompilationSection compilationSection) +8913774
   System.Web.HttpRuntime.HostingInit(HostingEnvironmentFlags hostingFlags) +152

[HttpException (0x80004005): The current identity (NT AUTHORITY\NETWORK SERVICE) does not have write access to 'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files'.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +8886319
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +85
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +259

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Version Information: Microsoft .NET Framework Version:2.0.50727.3053; ASP.NET Version:2.0.50727.3053

This is due to the Network Service Account not having write permissions to the temporary directory.

This can be resolved by adding the permissions via Windows Explorer, via CACLS:

CACLS "%windir%\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files" /E /T /G "NT AUTHORITY\NETWORK SERVICE:C"

or by running aspnet_regiis.exe with the -ga parameter:

%windir%\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis.exe -ga "NT AUTHORITY\NETWORK SERVICE"

How to Disable the Default Error Message in Web Interface 3.0
http://support.citrix.com/article/CTX103673