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: