Changing the web.config takes the WCF service down?

I have a simple WCF service hosted on IIS. Anytime I change the web.config file the service goes down and shows the following error:

Could not load file or assembly 'AutoMapper, Version=1.1.0.188, Culture=neutral, PublicKeyToken=be96cd2c38ef1005' or one of its dependencies. Logon failure: unknown user name or bad password. (Exception from HRESULT: 0x8007052E)

To correct it, I need to delete all existing files and re-publish the WCF service. Re-publishing without deleting old files doesn't work, and neither does restarting the iis server.

Does anyone know why this happens? Or if there is an alternative to republishing the service to bring it back up? I am using IIS on Windows Server 2003, which I think is 6.0, and .Net framework 4.0

Further Error Info...

Assembly Load Trace: The following information can be helpful to determine why the assembly 'AutoMapper, Version=1.1.0.188, Culture=neutral, PublicKeyToken=be96cd2c38ef1005' could not be loaded.

WRN: Assembly binding logging is turned OFF. To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1. Note: There is some performance penalty associated with assembly bind failure logging. To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Stack Trace:

[FileLoadException: Could not load file or assembly 'AutoMapper, Version=1.1.0.188, Culture=neutral, PublicKeyToken=be96cd2c38ef1005' or one of its dependencies. Logon failure: unknown user name or bad password. (Exception from HRESULT: 0x8007052E)]
System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0
System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +39
System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks) +132
System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +144 System.Reflection.Assembly.Load(String assemblyString) +28
System.ServiceModel.Activation.ServiceHostFactory.CreateServiceHost(String constructorString, Uri[] baseAddresses) +208
System.ServiceModel.HostingManager.CreateService(String normalizedVirtualPath) +1440
System.ServiceModel.HostingManager.ActivateService(String normalizedVirtualPath) +44
System.ServiceModel.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) +615

[ServiceActivationException: The service '/Services/ClientLoginService.svc' cannot be activated due to an exception during compilation. The exception message is: Could not load file or assembly 'AutoMapper, Version=1.1.0.188, Culture=neutral, PublicKeyToken=be96cd2c38ef1005' or one of its dependencies. Logon failure: unknown user name or bad password. (Exception from HRESULT: 0x8007052E).]
System.Runtime.AsyncResult.End(IAsyncResult result) +679246
System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) +190
System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context, String routeServiceVirtualPath, Boolean flowContext, Boolean ensureWFService) +234 System.ServiceModel.Activation.HttpModule.ProcessRequest(Object sender, EventArgs e) +355
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75