Version 5.1 has a problem with password which has been overcome in the 5.1.1CE-Version and in the github master. The problem exists only for users who use password_hash instead of sha512 as security.
Issue of Github : If You have no more access, You can edit "/data/_site/users.php" and change 'passhash' => 'password_hash' to 'passhash' => 'sha512' for every user. Then 'renew lost password' should function ( users should not use password_hash as security : functions only in the CE-Version and in the master) . There You find only the hash of a password.
You can also rename the /include-directory to include.old and replace it with the include-directory of the CE-Version. Then You should be able to change the password online (and are updated for php7.3).