If a user tries to login on kdm with expired password, the user is forced to change his password immediately, which is the expected behavior. But if the user cancels that password change diolog, the login form on the kdm can't be submitted anymore, so no login possible for any user.
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/