Sharepoint search error The item could not be indexed successfully because the item failed in the indexing subsystem.

https://docs.microsoft.com/en-us/archive/blogs/sp/clear-sharepoint-config-cache-with-powershell

http://bresleveloper.blogspot.com/2016/01/solved-item-could-not-be-indexed.html

in general

https://docs.microsoft.com/en-us/archive/blogs/sp/
https://blogs.msdn.microsoft.com/sharepoint_strategery/2014/12/04/sp2013-understanding-storage-locations-for-files-gathered-by-the-crawl-component/

is a good Knowledge base to keep in mind!