When logging into Joomla! administrator, the user is redirected back to the login page. If the username/password are valid, no error messages. If the username/password are invalid, error messages to the effect.
The following is found in the Apache access log:
::1 - - [timestamp] "POST /localhost/administrator/index.php HTTP/1.1" 303 -
::1 - - [timestamp] "GET /localhost/administrator/index.php HTTP/1.1" 200 6969
This is on the local copy of the site. It has not been touched since August 2018. The problem also occurs on the live site. Other Joomla! sites on localhost do work.
Have checked the #__extensions table and all necessary extensions are enabled. com_login, com_users, plg_authentication_joomla, plg_authentication_cookie, plg_user_joomla, etc.
Excerpts from config.php:
public $live_site = '';
public $cookie_domain = '';
public $cookie_path = '';
PHP session.save_path is server writable.
Try disabling Search Engine Friendly URLs and URL Rewriting in Global Configuration / configuration.php i.e.:
public $sef = '1';
public $sef_rewrite = '1';
Try temporarily renaming .htaccess to .htaccess.old to see if this makes any difference.
Try clearing the Joomla cache, any server side cache (e.g. LiteSpeed Cache) and clearing your web browser cache.
You may also need to update the tmp and logs files paths in Global Configuration / configuration.php although this probably isn't causing the problem you are experiencing.
Another thing to try is a different PHP version.