Troubleshooting a Failed Opening Required file Error
Symptom
When accessing parts of WHMCS, you may see the following Oops error:
main(): Failed opening required '/path/to/whmcs/..filename.php'...
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Cause
A required file is missing (in this example, /whmcs/includes/orderfunctions.php).
Resolution
Upload the missing file to the folder path in the error message. We recommend re-uploading of all the WHMCS files to ensure that no other files are missing. Make certain that you successfully overwrite all of WHMCS's files.
The Full Release files can be downloaded from the WHMCS Downloads page.