This error comes back time-to-time, especially after a Joomla site is moved from one server to another. The symptoms are simple - but frightening. You try to login to the administrator interface as usual, with your well known credentials, and the login box is reloading whitouth any kind of error message and withouth letting you in. No errors in browser console, no error in the logs, and no errors even if you turn the error reporting to maximum or the Joomla debug on. You can try even the tricks described elsewhere in this site to change your admin password - that won't help.
Why? Because the cause(s) - yeah, there can be multiple causes - are different from having bad credentials! Let's see what you can do about!
ATTENTION: these tricks are qualified as core hacks, so be careful: backup, wear your lucky hat, keep your finger crossed - and take any other security measures before you proceed - you can ruin your site easily with a wrong move.