The error is due to corrupted .htaccess configuration. Please backup the old .htaccess file and create new one with the following rules.
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]
You can also refer “Multisite” section in the following page to get correct .htaccess file.
https://codex.wordpress.org/htaccess
Thanks @InterServer, it worked
