Internal server error after enabling password

I’m working on a site where we need the whole site to be behind a password. It works fine until I enable password protection. Loads properly, even if the plugin is installed (but not enabled).

Once I enable password protection though, I start getting a “500 internal server error” every time I enter the password to access the site. Disabling the password bring everything back to normal.

I’ve tried deleting the .htaccess file. Doesn’t help. Any other ideas?

https://ww.wp.xz.cn/plugins/password-protected/