To troubleshoot, first try setting your control panel sessions to be cookie based only:
Add this to /system/expresionengine/config/config.php
$config['cp_session_type'] = 'c';
If that doesn't work, try disabling extensions via your config file:
$config['allow_extensions'] = 'n';
If that allows you to login, it's likely an add-on that's redirecting to a URI that's either invalid or redirected (thereby rejecting your POST data). You can confirm/observe this by viewing your "Network" log in your Developer Tools.
For me, the easiest solution is to just disable redirects on control panel login. You can either delete the cp_member_login hooks from the exp_extensions database table or try updating Structure (or the suspected add-on) to not redirect on login. Note that it's possible that you have more than one app trying to redirect (e.g. Republic Analytics, Structure, etc)