403 Forbidden
- Mod Security (most common): click here for instructions
- Incorrect include file location
- Other apache rules or permissions
For more details please check apache logs.

500 Internal Server Error
Issue related to the server configuration, examples, and most common issues:
- Domain has custom PHP version configured using .htaccess and that PHP version isn't installed
- Domain has defined Options in .htaccess file, you need to remove line starting with Options
- PHP/PHP-FPM version is not working well with the site code, try another version.
- Error: "End of script output before headers" this can be related to file permissions, try using fix account permissions

502 Bad Gateway
- PHP-FPM is down, try to restart PHP-FPM service
- It can be also related to the small number of php-fpm processes, this can be changed from WebServers_conf by using "PHP-FPM default template" default process limit is 4.
if some have issue increase to 20 or more depending on the requirements
- It can be also related to apache having all processes used if you use nginx, in this case, if the server is not overloaded apache number of the process will need to be increased.

503 Service Unavailable
- This is probably related to incorrect port redirection or php-fpm socket if not working
- php-fpm is down
- php-fpm specific version not installed (example missing php-fpm56 and domain is set to use it)
Error_Log: FCGI: attempt to connect to Unix domain socket /opt/alt/php-fpm56/usr/var/sockets/wwwener.sock (*) failed

Rebuild webservers and tail log: /var/log/cwp/webservers.log
2018-12-20 01:12:57 PHP-FPM php-fpm72: ERROR failed at quota or security limits
TEST

# su USERNAME -c "touch /home/USERNAME/.quota-check" -s /bin/bash
su: cannot create child process: Resource temporarily unavailable
su: cannot open session: System error

then this is probably related to users security limits you need to increase it and don't use 0 as this is the lowest limit.

if we have this output

touch: cannot touch '/home/USERNAME/.quota-check': Permission denied

then this means users home folder has incorrect permissions /home/USERNAME/

it can be also related to session restrictions in the pam file
/etc/pam.d/su [know platforms having this issue: lxckvm]
change "session include system-auth" to "session optional system-auth"

503 Backend fetch failed
- This is probably related to incorrect backend configuration for varnish

https://stackoverflow.com/questions/34548690/how-to-increase-timeout-between-varnish-4-and-nginx

504 GATEWAY TIMEOUT
- probably related to apache or php-fpm not responding, try restarting apache or php-fpm version you are using

For Apache issue: If the issue repeats then try to increase apache limits
For PHP-FPM: If the issue repeats then try to increase the process limit as described above
...more info to be added here

To solve the issue try disabling any redirect plugin/module you've installed for your website and check if your website is loading fine or not.

If it still doesn't solved alternatively you can also try to add this .htaccess rule by removing/replacing current http-https redirect rule from .htaccess :

SetEnvIf X-Forwarded-Proto "https" HTTPS=on
RewriteEngine on
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP:X-Forwarded-Proto} !https [NC]
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Step 1 :

Check Nginx web server/Proxy is already installed on your server

Step 2 :

Download and install Nginx Static Brotli module :

cd /etc/nginx/modules
wget http://dl1.centos-webpanel.com/files/nginx/modules/nginx-brotli-modules.zip
unzip nginx-brotli-modules.zip
rm -rf nginx-brotli-modules.zip 

Step 3 :

Now add nginx module configuration on "nginx.conf" :

nano /etc/nginx/nginx.conf

Now add this lines on top of the nginx.conf :

load_module "modules/ngx_http_brotli_filter_module.so";
load_module "modules/ngx_http_brotli_static_module.so";

Now add brotli compression configuration in nginx.conf file in http {section and before http closing }:

   #brotli
    brotli              on;
    brotli_comp_level   6;
    brotli_static       on;
    brotli_types        text/xml image/svg+xml application/x-font-ttf image/vnd.microsoft.icon application/x-font-opentype application/json font/eot application/vnd.ms-fontobject application/javascript font/otf application/xml application/xhtml+xml text/javascript  application/x-javascript text/plain application/x-font-truetype application/xml+rss image/x-icon font/opentype text/css image/x-win-bitmap;

Save the file and restart nginx :

service nginx restart
or
systemctl restart nginx

Go to one of the site to check :
https://www.giftofspeed.com/gzip-test/
https://www.websiteplanet.com/webtools/gzip-compression/

For advanced user you can check content-encoding via http header :

HTTP/2.0 200 OK
server: nginx
content-type: text/html; charset=UTF-8
vary: Accept-Encoding, Cookie
content-encoding: br