After updating to WHMCS 8.6.1, you may experience an Undefined Constant error when viewing the Monthly Orders report in PHP 8.1 environments:

Error: Undefined constant "tbladdons" in /path/to/whmcs/modules/reports/monthly_orders.php:81

We have created a hotfix for WHMCS 8.6.1 to resolve the error:

https://whmcs.community/files/file/183-module-7874-typeerror-in-processcreditcardpayments-task-with-stripe/

Upgrade your hosting environment to PHP 7.4.