This error is usually related with the permissions/ownership issue.

suPHP requires that the files are owned by the user name, and suExec requires that they are executed from the correct home folder.

Permissions on php files should also be set to 640 (-rw-r-----), folder permission must be 755.
All other files like html, css, js and others should have 644.

How to change ownership on the files and folders
go to CWP.admin Left Menu --> User Accounts -- > Fix Permissions and select user.

Apache Logs are in:

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 on the server.
- 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.

SoftException ERROR related to /opt/alt/

SoftException in Application.cpp:542: Could not execute script "/home/USERNAME/public_html/index.php"
Caused by SystemException in API_Linux.cpp:444: execve() for program "/opt/alt/php56/usr/bin/php-cgi" failed: Permission denied
End of script output before headers: index.php

* This is related to expired CWPpro license

Check more detailed info about webserver errors here.