Password protected pages result in 404 error

Which plugin?

Does it work on subdomains that aren’t mapped?

I’m not using a plugin for password protected pages. The domain mapping plugin is:

domain_mapping.php
Plugin Name: WordPress MU Domain Mapping
Plugin URI: https://ocaoimh.ie/wordpress-mu-domain-mapping/
Description: Map any blog on a WordPress MU website to another domain.
Version: 0.3
Author: Donncha O Caoimh

To answer your question, “Does it work on subdomains that aren’t mapped?” I tried and it did. Then I changed the name of the page I originally protected (on the mapped domain) and it now works again!

I meant which domain mapping plugin ??

You need to upgrade the domain mapping plugin

https://www.ads-software.com/extend/plugins/wordpress-mu-domain-mapping/