Hello,
I've just made a test with PHP 7.1.20 and the plugin has worked as except.

Could you please send a screenshot of the error that you obtain by using the plugin, and explain how to replicate the issue?

Thanks
Kind regards

Hello,
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

Let me know, thanks