Last week we had a very strange problem with the admin login of a Magento client of ours. When we tried to login we would get a session id in the url and everything seemed to work, but we were redirected to the same login box.
When you tried wrong password again and again then magento2 itself locks the user. So just try to unlock the user by following commands from command line (execute this command from your magento2 root directory) php bin/magento admin:user:unlock {user name} After executing this command you will be able to login and now you can set again the new password.
