I'm currently looking to get a better understanding of some of the errors that umbraco saves in its log files.

And I've now seen the following two errors pop up quite a few times, and haven't been able to figure out the actual cause of the errors. So I'm hoping that someone here might have a better insight into the errors and how to deal with them.

The umbraco version is 7.2.1

First error:

ERROR Umbraco.Core.UmbracoApplicationBase - [Thread 32] An unhandled exception occurred System.InvalidOperationException: Sequence contains no elements at System.Linq.Enumerable.First[TSource](IEnumerable`1 source) at Umbraco.Core.Configuration.UmbracoSettings.ContentErrorsElement.get_Error404Collection() at umbraco.library.GetCurrentNotFoundPageId() at Umbraco.Web.Routing.ContentFinderByLegacy404.TryFindContent(PublishedContentRequest pcr) at Umbraco.Web.Routing.ContentLastChanceFinderByNotFoundHandlers.HandlePageNotFound(PublishedContentRequest docRequest) at Umbraco.Web.Routing.ContentLastChanceFinderByNotFoundHandlers.TryFindContent(PublishedContentRequest docRequest) at Umbraco.Web.Routing.PublishedContentRequestEngine.HandlePublishedContent() at Umbraco.Web.Routing.PublishedContentRequestEngine.UpdateRequestOnMissingTemplate() at Umbraco.Web.Routing.PublishedContentRequest.UpdateOnMissingTemplate() at Umbraco.Web.Mvc.RenderRouteHandler.GetHandlerForRoute(RequestContext requestContext, PublishedContentRequest publishedContentRequest) at System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(HttpContextBase context) at Umbraco.Web.UmbracoModule.ProcessRequest(HttpContextBase httpContext) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Second error:

ERROR Umbraco.Web.Scheduling.ScheduledPublishing - [Thread 28] An error occurred with the scheduled publishing System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a receive. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) --- End of inner exception stack trace --- at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.Connection.SyncRead(HttpWebRequest request, Boolean userRetrievedStream, Boolean probeRead) --- End of inner exception stack trace --- at System.Net.WebClient.UploadDataInternal(Uri address, String method, Byte[] data, WebRequest& request) at System.Net.WebClient.UploadString(Uri address, String method, String data) at Umbraco.Web.Scheduling.ScheduledPublishing.Run()