Hello Vivian,
Thank you, I have double checked all the above (step 1,2).
Regarding step 3: The Site Collection Search Center does not return any results for this site content.
This is the Site Collection Search Center
Site Content: Metadata and Document Content Type
Site Collection Search Center Return no result from Site content
It looks to me as if the search crawler indexer has stopped working since october for this specific sitecolection.
Please advice!
Regards,
Nezah Samur