OData WCF service suddenly starts to return 404 on .Net 4.0 - wcf

Question

We have an OData WCF service running on some customers servers. Everything worked very well on .Net 3.5, but after having upgraded our solution to .Net4, the WCF service suddenly starts returning 404 for all requests after random time intervals, usually several days.
The rest of the web site is running normally. The 404's appear to be happening after the website has restarted.
In order to make it work again, we have make a change in web.config. Calling HttpRuntime.UnloadAppDomain(); fails to fix the problem.
We're trying to reproduce the error locally, but since it sometimes takes several days before it happens, it's a difficult problem to reproduce.
Any suggestions?
EDIT:
This is the error message from the server event log:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="System.ServiceModel 4.0.0.0" />
<EventID Qualifiers="49154">3</EventID>
<Level>2</Level>
<Task>5</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2012-06-02T11:15:32.000000000Z" />
<EventRecordID>28236</EventRecordID>
<Channel>Application</Channel>
<Computer>xxxx-ST-001</Computer>
<Security UserID="S-1-5-82-3945269471-185734930-3707521103-2442450153-1755958218" />
</System>
- <EventData>
<Data>System.ServiceModel.Activation.HostedHttpRequestAsyncResult/65092316</Data>
<Data>System.Web.HttpException (0x80004005): There was no channel actively listening at 'http://11.11.11.11/waf/services/OData.svc'. This is often caused by an incorrect address URI. Ensure that the address to which the message is sent matches an address on which a service is listening. ---> System.ServiceModel.EndpointNotFoundException: There was no channel actively listening at 'http://11.11.11.11/waf/Services/OData.svc'. This is often caused by an incorrect address URI. Ensure that the address to which the message is sent matches an address on which a service is listening. at System.ServiceModel.Activation.HostedHttpTransportManager.HttpContextReceived(HostedHttpRequestAsyncResult result) at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.HandleRequest() at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.BeginRequest() at System.Runtime.AsyncResult.End[TAsyncResult](IAsyncResult result) at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result)</Data>
<Data>w3wp</Data>
<Data>1852</Data>
</EventData>
Update 18.december 2012:
We're able to reproduce the error locally by making sure the first request to the site after a restart is made to the WCF OData service. The error message:
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
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:
[NullReferenceException: Object reference not set to an instance of an object.]
System.Web.Compilation.DiskBuildResultCache.CacheBuildResult(String cacheKey, BuildResult result, Int64 hashCode, DateTime utcStart) +9508708
System.Web.Compilation.BuildManager.CacheBuildResultInternal(String cacheKey, BuildResult result, Int64 hashCode, DateTime utcStart) +70
System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) +9929866
System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +299
System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +103
System.Web.Compilation.BuildManager.GetVPathBuildResult(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean ensureIsUpToDate) +82
System.Web.Compilation.BuildManager.GetCompiledCustomString(String virtualPath) +29
System.ServiceModel.HostingManager.GetCompiledCustomString(String normalizedVirtualPath) +164
System.ServiceModel.HostingManager.CreateService(String normalizedVirtualPath, EventTraceActivity eventTraceActivity) +206
System.ServiceModel.HostingManager.ActivateService(ServiceActivationInfo serviceActivationInfo, EventTraceActivity eventTraceActivity) +52
System.ServiceModel.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath, EventTraceActivity eventTraceActivity) +598
[ServiceActivationException: The service '/ecoodata' cannot be activated due to an exception during compilation. The exception message is: Object reference not set to an instance of an object..]
System.Runtime.AsyncResult.End(IAsyncResult result) +495736
System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) +178
System.ServiceModel.Activation.ServiceHttpModule.EndProcessRequest(IAsyncResult ar) +348782
System.Web.AsyncEventExecutionStep.OnAsyncEventCompletion(IAsyncResult ar) +9663393
Anyone got a clue what this might be?