1. If you have changed the admin URL in your WHMCS installation.
In this case, whenever you are redirected to the WHMCS admin URL in Monitis WHMCS plugin you will get the 404 “Page Not Found” error, since Monitis plugin uses the standard non-customized admin URL for WHMCS.
To solve this problem, open the lib/constants.php file, find the below command and replace /admin with whatever is the actual admin URL path that you use in your WHMCS installation:
define (MON_ADMIN_PATH, “/admin”);
2. In case the /modules directory is not located in the root of your whmcs installation, you need to modify accordingly some of the paths in Monitis WHMCS addon source codes.
To do that open the hooks.php and montis.php files from the monitis addon directory, find and replace the paths containing the /modules directory with the actual paths according to your directory structure.