I have had the same issue on each multi-site I’ve installed.
When you set up the multisite network for WordPress you were asked to copy some code into the .htaccess file.
Change the following line:
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
to:
RewriteRule (wp-(login|content|admin|includes).*) $1 [L]
The first line will ONLY work on the main site url. The second line adds wp-login to the list of acceptable wp folders/files and loads them relative to the root. The problem with this is rule is that it does not matter where these components appear in the url – you can write a blog post with the url '/how-to-use-wp-admin/' and the website will internally attempt to redirect you to '/wp-admin/'. What are the chances of that, though?