WHMCS will display this message when it encounters a PHP issue.
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WHMCS error
There are two methods to check the issue details. The first one is to look for details in the error_log file in the WHMCS folder. The other one is to set WHMCS to display such error details in web pages.
To display PHP errors in web pages:
1. Log in to WHMCS installation as an admin
2. Navigate to Setup -> General Settings ; Other tab
3. Here enable the Display Errors option and click the Save Changes button
Now you will see the details in the webpages as below:
Resources:
WHMCS Oops help page