I go this error when I tried to crawl a web application (2013 upgrade to 2016):
"Access is denied. Verify that either the Default Content Access Account has access to this repository, or add a crawl rule to crawl this repository. If the repository being crawled is a SharePoint repository, verify that the account you are using has "Full Read" permissions on the SharePoint Web Application being crawled."
I already did
(1) The "Default content access account" has full access to the site collection. It also in permission to the web application.
(2) The account has access (administrator & full accesses) to "User Profile Service Application" and "Search Service Application".
(3) "DisableLoopbackCheck" and "BackConnectionHostNames" already added to the server register.
Please let me know if something else I should look for. Many thanks.