If you cannot login WHMCS dashboard after migration or some server’s change like PHP update
The admin page may resets and doesn’t allow you to login
First of all I advise you to check if you have correct session.save_path inside yo PHP settings
Then one of possible problem may be outdated PHP settings like below:
php_value session.save_path "/var/cpanel/php/sessions/ea-php70"
You need to check your .htaccess, php.ini, .user.ini to be sure they are all good