If you are encountering the login problem at front end and admin panel, one possible reason is that you set a too bigger value for post_max_size in php.ini.

In practice, we found post_max_size should not be bigger then 2GB =2048MB. Otherwise, PHP would not parse correctly the raw data sent from client browsers. This raise the issues on logging in sites, since your input data(such as username, password) are not recognized by PHP.

So the solution is to decrease post_max_size to strictly less then 2048MB in php.ini