This question has been answered by paulp. See the first response.
Could there be a .htaccess file in the Manager folder that is causing this?
RewriteEngine Off
RewriteEngine On
RewriteBase /
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://yoursite.com/manager/ Also from the docs, try the following in you manager .htaccess file
RewriteEngine On RewriteBase / RewriteCond %{SERVER_PORT} 80 RewriteRule ^(.*)$ https://yoursite.com/manager/
https://docs.modx.com/revolution/2.x/administering-your-site/security/hardening-modx-revolution
I moved one over to https a couple of days ago, try changing the link_tag_scheme system setting from -1 to https and make sure you have deleted everything in the core >> Cache folder and clear all browser cache
Also from the docs, try the following in you manager .htaccess file
RewriteEngine On RewriteBase / RewriteCond %{SERVER_PORT} 80 RewriteRule ^(.*)$ https://yoursite.com/manager/
https://docs.modx.com/revolution/2.x/administering-your-site/security/hardening-modx-revolution
The htaccess file in the manager folder will only affect the manager and sub folders of the manager, so if everything is working without it there is no need for it.