我的网络应用程序以前工作正常。突然，今天早上我尝试重建我的应用程序，然后发生了一些错误。 Application构建成功，但是当我使用浏览器打开它时，会发生以下错误：
Server Error in '/MyWebAppName' Application.
[C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll] The signature is incorrect.
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.BadImageFormatException: [C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll] The signature is incorrect.
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:
[BadImageFormatException: [C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll] The signature is incorrect.]
[BadImageFormatException: Could not load file or assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. An attempt was made to load a program with an incorrect format.]
System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0
System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +210
System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection) +242
System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +17
System.Reflection.Assembly.Load(String assemblyString) +35
System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +122
[ConfigurationErrorsException: Could not load file or assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. An attempt was made to load a program with an incorrect format.]
System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +12761078
System.Web.Configuration.AssemblyInfo.get_AssemblyInternal() +214
System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +334
System.Web.Compilation.BuildManager.CallPreStartInitMethods(String preStartInitListPath) +203
System.Web.Compilation.BuildManager.ExecutePreAppStart() +152
System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException) +1151
[HttpException (0x80004005): Could not load file or assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. An attempt was made to load a program with an incorrect format.]
System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +12881540
System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +159
System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +12722601
Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17929
环境Visual Studio 2012，Windows 7 64位。
我的构建目标是AnyCPU。我尝试使用构建目标重建为x86，失败。然后更改为构建目标x64，也失败了。
实际上有什么问题？