C# net core Odata web api json format exception

I would like to return an entity (“Question5Point1”) with a collection of complex objects from C# net 5 web api. My problem is, that my json result is cut at the beginning of the complex object. It’s looks like that: enter image description here

My metadata looks like that:

enter image description here

My code looks like that:

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The result would be:

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Every field/attribute is filled. Why this error happens and what can i do against it? In the output of debugging, I see this exception:

    Microsoft.OData.ODataException: An internal error 'EdmLibraryExtensions_ToTypeReference' occurred.
   at Microsoft.OData.Metadata.EdmLibraryExtensions.ToTypeReference(IEdmType type, Boolean nullable)
   at Microsoft.OData.Metadata.EdmLibraryExtensions.ToTypeReference(IEdmType type)
   at Microsoft.OData.TypeNameOracle.ResolveAndValidateTypeFromTypeName(IEdmModel model, IEdmStructuredType expectedType, String typeName, IWriterValidator writerValidator)
   at Microsoft.OData.ODataWriterCore.GetResourceType(ODataResourceBase resource)
   at Microsoft.OData.ODataWriterCore.ValidateResourceForResourceSet(ODataResourceBase resource, ResourceBaseScope resourceScope)
   at Microsoft.OData.ODataWriterCore.<>c__DisplayClass121_0.<WriteStartResourceImplementation>b__0()
   at Microsoft.OData.ODataWriterCore.InterceptException(Action action)
   at Microsoft.OData.ODataWriterCore.WriteStartResourceImplementation(ODataResource resource)
   at Microsoft.OData.ODataWriterCore.<>c__DisplayClass49_0.<WriteStartAsync>b__0()
   at Microsoft.OData.TaskUtils.GetTaskForSynchronousOperation(Action synchronousOperation)
--- End of stack trace from previous location ---
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSerializer.WriteResourceAsync(Object graph, ODataWriter writer, ODataSerializerContext writeContext, IEdmTypeReference expectedType)
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSetSerializer.WriteResourceSetAsync(IEnumerable enumerable, IEdmTypeReference resourceSetType, ODataWriter writer, ODataSerializerContext writeContext)
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSerializer.WriteComplexAndExpandedNavigationPropertyAsync(IEdmProperty edmProperty, SelectItem selectItem, ResourceContext resourceContext, ODataWriter writer)
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSerializer.WriteComplexPropertiesAsync(SelectExpandNode selectExpandNode, ResourceContext resourceContext, ODataWriter writer)
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSerializer.WriteResourceAsync(Object graph, ODataWriter writer, ODataSerializerContext writeContext, IEdmTypeReference expectedType)
   at Microsoft.AspNet.OData.Formatter.Serialization.ODataResourceSerializer.WriteObjectAsync(Object graph, Type type, ODataMessageWriter messageWriter, ODataSerializerContext writeContext)
   at Microsoft.AspNet.OData.Formatter.ODataOutputFormatterHelper.WriteToStreamAsync(Type type, Object value, IEdmModel model, ODataVersion version, Uri baseAddress, MediaTypeHeaderValue contentType, IWebApiUrlHelper internaUrlHelper, IWebApiRequestMessage internalRequest, IWebApiHeaders internalRequestHeaders, Func`2 getODataMessageWrapper, Func`2 getEdmTypeSerializer, Func`2 getODataPayloadSerializer, Func`1 getODataSerializerContext)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|29_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNet.OData.Batch.ODataBatchMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Server.IIS.Core.IISHttpContextOfT`1.ProcessRequestAsync()