I've just tried with PHP 7.2, as well the plugin has worked correctly.
In the plugin source code there is no "mcrypt_decrypt" PHP function. May you have another plugin which interferes with the login process?
You can try to disable all your third-party plugins except our, then try again to login by the URL in the frontend, by this URL:
http://YOUR SITE NAME/index.php?username=YOUR-USERNAME&password=YOUR-PASSWORD