I activated multisite using subdirectories on my old WordPress site, and I created a new site (say my-example/new-site/).

I found links such as my-example/new-site/wp-content/mu-plugins/css/ can redirect to my-example/wp-content/mu-plugins/css/, however, my-example/new-site/wp-content/mu-plugins/css/style.css won't redirect and will throw out a 404 error, this leads to the site loading errors.

I was so confused, and tried hard refreshing sites, clearing cookies, but it didn't work.

The .htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

I activated multisite using subdirectories on my old WordPress site, and I created a new site (say my-example/new-site/).

I found links such as my-example/new-site/wp-content/mu-plugins/css/ can redirect to my-example/wp-content/mu-plugins/css/, however, my-example/new-site/wp-content/mu-plugins/css/style.css won't redirect and will throw out a 404 error, this leads to the site loading errors.

I was so confused, and tried hard refreshing sites, clearing cookies, but it didn't work.

The .htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

• redirect
• htaccess
• rewrite-rules
• 404-error
• cache

• "my-example/new-site/wp-content/mu-plugins/css/ can redirect to my-example/wp-content/mu-plugins/css/" - I'm not sure what you mean by that? There should be no "redirect" occurring here? – MrWhite Commented May 8, 2019 at 10:53
• 1 @MrWhite According to rule RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L], the redirect will occur. – Guoyang Qin Commented May 8, 2019 at 17:53

1 Answer 1

After trying a million of ways, I contacted the super admin and asked for permission to log into WP Engine, our host. I found there is a Multisite setting too. So I clicked "convert to multisite", and, bang! the error is resolved. Subsites can work properly now.