PHP and WHMCS
Your PHP configuration is extremely important for successfully running WHMCS.
For more information about PHP and WHMCS, see:
500 Internal Server Error
The 500 internal server error indicates there is a problem processing the request. This is usually due to a customisation or an issue with the PHP configuration causing issues.
The first step would be to try and get further details on the error being raised. You can do this by adding $display_errors = true; to your configuration.php file or reviewing the PHP error_log file.
This should indicate which file is causing the error. If it is a 3rd party module or customization then you should remove the custom files and confirm if this resolves the error.
Maximum Execution Time
By default, WHMCS enforces a max_execution_time limit of 300 seconds for general requests.
If this limit is too little for a given task or operation, you may encounter an error message that looks something like the following
PHP Fatal error: Maximum execution time of 300 seconds exceeded in /path/to/file.php
Increasing the max_execution_time
If you wish to override the max execution time limit and increase it to a higher value, you can do so by adding the following line to your WHMCS configuration.php file:
$overidephptimelimit = 500;