You marked my previous question as resolved but it is not. Pls test this !
You cant login in a wpms with your rewrite code when you are using mapped domain which also have the original subdomain path completely hidden. You can get to the login page. But after entering the password you get the 404 and are not logged in.
It does not work with mapped domains in wpms unless you remove the root domainurl from the htaccess rewrite code and replace it by a *
Pls set it up mapdomain and try. It does allow you to get to the login screen but it does not let you login. Mapped domains act as a root domain not as a subdomain. Especially when you also haven hidden the subdomain completely by changing that url to the mapped domain url also in the site settings of that (sub)domain,
Its not a child domain anymore.
Please test this.