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

During various search troubleshooting i came across the following crawling error in the Crawl log of a SharePoint 2013 environment.

Processing this item failed because of an unknown error when trying to parse its contents. (Error parsing document ‘http://********.*****.com/Project/abcd/Q_M/ABX/SitePages/Homepage.aspx’. Sandbox worker pool is
closed.; ; SearchID = *******************)

In order to fix this you can try to perform the following action plan:
Open “Local Policies
Click on “User rights assignment

user-rights-assignment

Make sure that the search service account has the following rights:
Replace a process level token

adjust-memory-quotas-for-process

Adjust memory quotas for a process

adjust-memory-quotas-for-process-properties

Impersonate a client after authentication

impersonate

Please make sure that the policies don’t get changed afterwards.

After implementing the above changes please run a clear configuration cache
After clearing the cache, start a full crawl and the errors should be gone.

Start SharePoint Service Application Proxy using Powershell

If your Usage and Health Data Collection Proxy is in a stopped state here is a quick bit of PowerShell to to get it started:

$sap = Get-SPServiceApplicationProxy | where-object {$_.TypeName -eq “Usage and Health Data Collection Proxy”}
$sap.Provision()

The above can easily be adapted to allow you to start any Service Application Proxy