I did an upgrade from a very old system (DNN 5.x) to DNN 9.5.

All seems ok except just a few things: pages, users and recycle bin. On these I get an API call error similar to this one:

    http://website/RunningOnF...tPageList?searchKey=

    {Message: "An error has occurred.",…}
        Message: "An error has occurred."
        ExceptionMessage: "The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'."
        ExceptionType: "System.InvalidOperationException"
        StackTrace: null
        InnerException: {Message: "An error has occurred.",…}
            Message: "An error has occurred."
            ExceptionMessage: "Object reference not set to an instance of an object."
            ExceptionType: "System.NullReferenceException"
            StackTrace: " en DotNetNuke.Entities.Tabs.TabInfo.get_FullUrl() ↵ en Dnn.PersonaBar.Pages.Components.Converters.ConvertToPageItem[T](TabInfo tab, IEnumerable`1 portalTabs) ↵ en System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext() ↵ en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) ↵ en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType) ↵ en Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType) ↵ en System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding) ↵ en System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding) ↵ en System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken) ↵--- Fin del seguimiento de la pila de la ubicación anterior donde se produjo la excepción --- ↵ en System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() ↵ en System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) ↵ en System.Web.Http.Tracing.ITraceWriterExtensions.d__24.MoveNext() ↵--- Fin del seguimiento de la pila de la ubicación anterior donde se produjo la excepción --- ↵ en System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() ↵ en System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) ↵ en System.Web.Http.WebHost.HttpControllerHandler.d__1b.MoveNext()"

Seems to come from this call get_FullUrl() but cannot find the issue there.

Any ideas?