Sharepoint Designer 2010 error

All of a sudden I was not able to open Intranet DEV site with Sharepoint Designer 2010. It was giving error "The Server could not complete your request" and in the eventlog the error was:

Log Name: Application Source: System.ServiceModel 3.0.0.0 Date: 1/24/2012 2:16:34 PM Event ID: 3 Task Category: WebHost Level: Error Keywords: Classic User: XXX Computer: XXX Description: WebHost failed to process a request. Sender Information: System.ServiceModel.ServiceHostingEnvironment+HostingManager/54467399 Exception: System.ServiceModel.ServiceActivationException: The service '/_vti_bin/client.svc' cannot be activated due to an exception during compilation. The exception message is: Operation is not valid due to the current state of the object.. ---> System.InvalidOperationException: Operation is not valid due to the current state of the object. at Microsoft.SharePoint.ApplicationRuntime.SPLayoutsMappedFile.MapLayoutsVirtualPath(String virtualPath) at Microsoft.SharePoint.ApplicationRuntime.SPVirtualPathProvider.GetCacheKey(String virtualPath) at Microsoft.SharePoint.Publishing.Internal.CmsVirtualPathProvider.GetCacheKey(String virtualPath) at System.Web.Compilation.BuildManager.GetCacheKeyFromVirtualPath(VirtualPath virtualPath, Boolean& keyFromVPP) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetCompiledCustomString(String virtualPath) at System.ServiceModel.ServiceHostingEnvironment.HostingManager.GetCompiledCustomString(String normalizedVirtualPath) at System.ServiceModel.ServiceHostingEnvironment.HostingManager.CreateService(String normalizedVirtualPath) at System.ServiceModel.ServiceHostingEnvironment.HostingManager.ActivateService(String normalizedVirtualPath) at System.ServiceModel.ServiceHostingEnvironment.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) --- End of inner exception stack trace --- at System.ServiceModel.ServiceHostingEnvironment.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) at System.ServiceModel.ServiceHostingEnvironment.EnsureServiceAvailableFast(String relativeVirtualPath) Process Name: w3wp Process ID: 8476

The solution was according to this post. In web.config just under the line: <system.servicemodel> add the following line: <servicehostingenvironment aspNetCompatibilityEnabled="true"/> after iisreset I was able to open a site.

update: there might be similar symptoms in case of low memory conditions. -the solution is obvoisly to add RAM.