I have a .net4 web application running in a 64 bits 2008 server. I can only get it to run when I set the app pool to Enable 32-bits application to true. All dlls are compiled for .net4 (verified with corflags.exe). How can I figure out why Enable 32-bit is required?

The error message from the event log when starting as a 64-bit app-pool

Event code: 3008 Event message: A configuration error has occurred. Event time: 2011-03-16 08:55:46 Event time (UTC): 2011-03-16 07:55:46 Event ID: 3c209480ff1c4495bede2e26924be46a Event sequence: 1 Event occurrence: 1 Event detail code: 0

Application information: Application domain: removed Trust level: Full Application Virtual Path: removed Application Path: removed Machine name: NMLABB-EXT01

Process information: Process ID: 4324 Process name: w3wp.exe Account name: removed

Exception information: Exception type: ConfigurationErrorsException Exception message: Could not load file or assembly 'System.Data' or one of its dependencies. An attempt was made to load a program with an incorrect format. at System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) at System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() at System.Web.Configuration.AssemblyInfo.get_AssemblyInternal() at System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) at System.Web.Compilation.BuildManager.CallPreStartInitMethods() at System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException)

Could not load file or assembly 'System.Data' or one of its dependencies. An attempt was made to load a program with an incorrect format. at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks) at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) at System.Reflection.Assembly.Load(String assemblyString) at System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective)

Request information: Request URL: "our url" Request path: "url" User host address: ip-adddress User:
Is authenticated: False Authentication Type:
Thread account name: "app-pool"

Thread information: Thread ID: 6 Thread account name: "app-pool" Is impersonating: False Stack trace: at System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) at System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() at System.Web.Configuration.AssemblyInfo.get_AssemblyInternal() at System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) at System.Web.Compilation.BuildManager.CallPreStartInitMethods() at System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException)

Custom event details: