Long shot, but have you applied any OS patches recently? There were some which corrupted IIS, so that might be the culprit.
I ran a search on this error in our case history. I checked on the most recent six or seven instances of this exact error and every single one was fixed by either resetting the indexing engine or by adding the proper antivirus exclusions and then resetting the indexing engine.
I hope that helps.
Thank you Chris appreciate it.
The index issue has coincided with the automatic install of the .Net Framework 4.7. I going to reinstall/repair .Net Framwork 3.5 and EV binaries if that does not work then I will opt for Migrate a new server with a fresh install.