Sometimes, users experience an issue whereby all pages display 404 errors after migrating to SSL. The cause may differ from user to user and from server to server.
Re-save the permalinks
Even though Really Simple Security flushes the internal rewrite rules, it sometimes helps to do this again. You can find the permalinks under Settings -> Permalinks. Re-saving makes sure the WordPress rewrite rules get flushed.
Photon from JetPack
Photon uses a CDN service to speed up loading for your images. When these image paths are changed, this might lead to issues. If you see the image paths are changed into wp1.com etc, try deactivating Photon.
.htaccess files in subdirectories
If you have .htaccess files in your media directories, remove them, or adapt them to the https domain.
Other redirecting plugins
Check also if you have a multi-language plugin, security, caching, or redirect plugins active. These especially multilanguage plugins often have strange effects when the URL has changed. Deactivate them, or re-save the settings, to flush the internal redirect rules of these plugins.
Hot-link protection on your server
Check if you have hot-link protection activated.
NGINX server settings
If you are on NGINX, there can be a different cause. Adding the following lines to the httpd-ssl.conf file often solves the issue.
<Directory C:/www-base/ >
Options FollowSymLinks
AllowOverride All
</Directory>
<Directory C:/www-base/site-dir >
Options Indexes FollowSymLinks MultiViews
AllowOverride All
Order allow,deny
allow from all
</Directory>This is a typical server configuration issue, so you might have to contact your hosting provider to fix it.