The publish at functionality doesn't work. The items simply stay unpublished forever. There are two errors in log that are somehow connected. Both are 401 errors. I tried to enable anonymous authenication to the folder Umbraco/Webservices, but that didn't help. I couldn't find any solution after hours of reading.

Error in ping. The base url used in the request was: ***, see http://our.umbraco.org/documentation/Using-Umbraco/Config-files/umbracoSettings/#ScheduledTasks documentation for details on setting a baseUrl if this is in error
System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   at System.Net.WebClient.DownloadDataInternal(Uri address, WebRequest& request)
   at System.Net.WebClient.DownloadString(Uri address)
   at Umbraco.Web.Scheduling.KeepAlive.Start(ApplicationContext appContext, IUmbracoSettingsSection settings)

An error occurred with the scheduled publishing. The base url used in the request was: ***, see http://our.umbraco.org/documentation/Using-Umbraco/Config-files/umbracoSettings/#ScheduledTasks documentation for details on setting a baseUrl if this is in error
System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   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.PerformRun()