SharePoint 2013: Processing this item failed because of an unknown error when trying to parse its contents

Problem

You perform a crawl, and then, checking the crawl log, you see an overwhelming number of errors like the following:

Processing this item failed because of an unknown error when trying to parse its contents...

and

The content processing pipeline failed to process the item...

Solution

1. Description: the Search Service account requires specific local user rights assignments to function fully, including:
1. Adjust memory quotas for a process
2. Impersonate a client after authentication
3. Replace a process level token
2. Short term: add the search service account (and the content access service account, if you use such) to the local Administrators group on each server in the farm hosting SharePoint Server.  By default, the members of the local Administrators group are automatically granted most all user rights, including those listed above.
3. Long-term: request your network administrators to modify your organization's GPO to grant your farm's Search Service account these rights.

References

• SharePoint 2013 search error ‘Sandbox worker is closed’
• Crawl error SharePoint 2013 - Processing this item failed because of an unknown error when trying to parse its contents
• Insufficient SQL Server database permissions - Event 5214 (SharePoint 2010 Products)
• Crawl errors: Sandbox worker pool is closed
• SharePoint 2013: Service Account Configurations and Permissions