This problem is caused when apache's mod_version module is not enabled. Unfortunately, my host does not allow me to enable this module and so I have devised the following workaround:
Open [magento_root]/.htaccess with your favourite text editor.
You will notice, towards the end of the file, there are numerous entries following this format:
Delete all the Products and Category URLs from Database table "url_rewrite" and then trying to save the each category.
Also check for the URL suffix from configuration.
Store > Configuration > Catalog > Catalog > Search Engine Optimization
Note you will need to remove the "IfVersion" commands from every .htaccess file in Magento, after the installation too. These are the files I had to modify with a fresh install Magento2 (2.4) and on the godaddy linux platform.
Login to the admin and go to
System > Configuration > Web and change the drop-down value for CMS No Route Page in the Default Pages section. You should be able to select your Home Page.
Second method by .htaccess:
For example, if you wish to redirect the 404 (File Not Found) errors to http://example.com, you should add this line to your .htaccess ...
My first guess is, that magento can't see, that you are surfing on your new store, instead of your old store under the category nested-store/about. Therefore the first thing you have to check is, whether magento uses the right storeview here.
If this is not the case, then you should add a vhost configuration wich sets the right website/storeview, like this:
Did you reindex and clean/flush the cache?
Try the basics first:
Make sure your virtual host settings are configured for ssl in apache config.
The file can be found here https://raw.githubusercontent.com/magento/magento2/2.0/app/code/Magento/Catalog/view/adminhtml/web/catalog/category-selector.css
Between release 2.0 and 2.1 the file disappears from the repository, so I imagine it's been deprecated in favor of a different solution.