A few things to check:
If you have wifi and a wired network connection, try disabling one, then the other. I have seen cases where a computer randomly switches between connections, invalidating the session immediately.
Have you renamed the /system folder? If so, make sure to update it in index.php and admin.php.
Make sure cp_url is set correctly in your config.php ESPECIALLY if you have renamed /system, but also if you have not. If you use admin.php to login set it like this:
$config['cp_url'] = 'http://example.com/admin.php'; // change example.com to your domain
If you use /system to login, then set like this:
$config['cp_url'] = 'http://example.com/system/'; // change example.com to your domain
$config['cp_session_type'] = 's';
$config['cp_session_type'] = 'c';
$config['cookie_secure'] = 'n';
If so, fix your SSL redirects so that you only ever serve pages via SSL, then remove the line.
$config['cookie_domain'] = '.example.com'; // change example.com to your domain, and remember to KEEP the initial period
$config['secure_forms'] = 'n';
$config['allow_extensions'] = 'n';
That’s about all I can remember / find about it.
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