I have received the errors on test site (XAMPP - localhost = SSL is disabled on the mainsite) and production site (Hosting provider = SSL is enabled on the mainsite) after upgrading the Pro Sites plugin few days ago. When trying to register/sign-up for a new site/blog, user was forced to the HTTPS and give me untrusted certificate page, and then 404 Object not found. The last time I tested the signup process was yesterday morning, it was working fine without any changes applied to the site afterwards. Please advice.
https://MainSite.localhost/pro-site/?action=new_blog&blog_template=X <-- XAMPP local
https://MainSite.com/pro-site/?action=new_blog&blog_template=X <-- Live Server