what could be this error

I always get this error occasionally...

"UILib" is a library(dll) attached to my project....
It was working fine and no problem...but sometimes occasionally I get this error
--------------------------------------------------------

Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: Access is denied: 'UILib'.

Source Error:

Line 196: <add assembly="System.EnterpriseServices, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
Line 197: <add assembly="System.Web.Mobile, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
Line 198: <add assembly="*"/>
Line 199: </assemblies>
Line 200: </compilation>

Source File: d:\winnt\microsoft.net\framework\v1.1.4322\Config\machine.config Line: 198

Assembly Load Trace: The following information can be helpful to determine why the assembly 'UILib' could not be loaded.

=== Pre-bind state information ===
LOG: DisplayName = UILib
(Partial)
LOG: Appbase = file:///d:/inetpub/wwwroot/eSMP
LOG: Initial PrivatePath = bin
Calling assembly : (Unknown).
===

LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
LOG: Post-policy reference: UILib
LOG: Attempting download of new URL file:///D:/WINNT/Microsoft.NET/Framework/v1.1.4322/Temporary ASP.NET Files/esmp/37394cef/fe96a4c0/UILib.DLL.
LOG: Attempting download of new URL file:///D:/WINNT/Microsoft.NET/Framework/v1.1.4322/Temporary ASP.NET Files/esmp/37394cef/fe96a4c0/UILib/UILib.DLL.
LOG: Attempting download of new URL file:///d:/inetpub/wwwroot/eSMP/bin/UILib.DLL.
LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
LOG: Post-policy reference: UILib, Version=1.0.1443.31586, Culture=neutral, PublicKeyToken=null