"Warning :Lucene Thread Stops" emails

"Warning :Lucene Thread Stops" emails

We are periodically getting bursts of 

Build No. : 7929

System Error Notification



Indexing (Lucene Thread ) is not running. Please delete the database entry (Task_Input entry) and restart the server to solve the problem


What might be causing this, and what is the correct way to fix it?

Thanks

Chris

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