What causing this error proxy_fcgi:error] AH01071: Got error 'Primary script unknown'

I'm getting the proxy_fcgi:error] AH01071: Got error 'Primary script unknown' error very frequently. Can anyone tell me what causing this error. What I learned so far is Rewrite rule may cause this issue. But do not understand exactly what causing this issue. Following are my Rewrite rules:

<VirtualHost *:80> ServerName example.com ServerAlias ServerAdmin webmaster@localhost DocumentRoot /var/www <Directory /var/www/> Options -Indexes +FollowSymLinks -MultiViews AllowOverride None Require all granted # Setting rewrite rules RewriteEngine on RewriteBase / RewriteCond %{HTTPS} off [OR] RewriteCond %{HTTP_HOST} !^www\.example\.com [NC] RewriteRule (.*) [R=301,L] # Show 404 error if there is any slash after page name RewriteRule ^.+?\.(php|html?)/ - [L,R=404,NC] # Custom Directory Index Files DirectoryIndex index.php index.html </Directory> </VirtualHost> <IfModule mod_ssl.c> <VirtualHost *:443> ServerName example.com ServerAlias ServerAdmin webmaster@localhost DocumentRoot /var/www <Directory /var/www/> Options -Indexes +FollowSymLinks -MultiViews AllowOverride None Require all granted # Setting rewrite rules RewriteEngine on RewriteBase / RewriteCond %{HTTP_HOST} ^example.com$ [NC] RewriteRule (.*) [R=301,L] # Show 404 error if there is any slash after page name RewriteRule ^.+?\.(php|html?)/ - [L,R=404,NC] # Custom Directory Index Files DirectoryIndex index.php index.html </Directory> Include /etc/letsencrypt/options-ssl-apache.conf SSLCertificateFile /etc/letsencrypt/live/ SSLCertificateKeyFile /etc/letsencrypt/live/ </VirtualHost> </IfModule> 

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