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