Content index state: Failed (The indexing [...] encountered an unexpected exception.)

Search on my Exchange 2013 is not working. Event 1009 with source MSExchangeFastSearch is logged in the Application log:

The indexing of mailbox database MB01 encountered an unexpected exception. Error details: Microsoft.Exchange.Search.Core.Abstraction.OperationFailedException: The component operation has failed. ---> Microsoft.Exchange.Search.Fast.FastConnectionException: Connection to the Content Submission Service has failed. ---> Microsoft.Ceres.External.ContentApi.ConnectionException: Given CSS specification failed:

Could not connect to CSS node at net.tcp://localhost:17028/ContentSubmissionServices/content with flow: Microsoft.Exchange.Search.WatermarkWriter.12.14. Error: Object reference not set to an instance of an object. Inner exception:---

   at Microsoft.Ceres.External.ContentApi.DocumentFeeder.DocumentFeeder.SetCssNodes(IEnumerable`1 newCssNodes)

   at Microsoft.Ceres.External.ContentApi.DocumentFeeder.DocumentFeeder..ctor(DocumentFeederOptions options)

   at Microsoft.Exchange.Search.Fast.FastFeeder.Initialize()

   --- End of inner exception stack trace ---

   at Microsoft.Exchange.Search.Fast.FastFeeder.Initialize()

   at Microsoft.Exchange.Search.Fast.Factory.InternalCreateFastFeeder(ISearchServiceConfig config, String indexSystemFlow, String indexSystemName, String instanceName, Int32 numberOfSessions)

   at Microsoft.Exchange.Search.Fast.FeederStateStorage.SendWatermarkUpdateToFast(Int64 fastId, Int64 watermark)

   at Microsoft.Exchange.Search.Fast.FeederStateStorage.SetMailboxCrawlerState(MailboxCrawlerState state)

   at Microsoft.Exchange.Search.Mdb.FeederStateInitializer.EnsureInitialStateSet()

   at Microsoft.Exchange.Search.Engine.SearchFeedingController.InternalExecutionStart()

   at Microsoft.Exchange.Search.Core.Common.Executable.InternalExecutionStart(Object state)

   --- End of inner exception stack trace ---

   at Microsoft.Exchange.Search.Core.Common.Executable.EndExecute(IAsyncResult asyncResult)

   at Microsoft.Exchange.Search.Engine.SearchRootController.ExecuteComplete(IAsyncResult asyncResult)>

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