We have checked with the Plesk team and confirmed that they are aware of the issue, and upgrading the Plesk server to 18.0.79 is recommended to resolve this.
Hi, @Ladylinux . According to the WHMCS team, their module is already intended to use 1.6.9.1 by design. So, I am not entirely sure where the error originates from in the first place. If you have an open case with support already, please ask them to further clarify that matter.
If for some reason you cannot update Plesk above 18.0.79, the issue could be mitigated on older versions by applying the following workaround:
However, that exposes a serious security risk and should be used as a last resort. Also, panel.ini changes are kept intact during the upgrades, so the change should be manually reverted after the upgrade.