If you are faced with this warning message as show below, this is caused by some Local Settings made in Administration.
This happens when trying to activate a prebooked visitor. See picture below.
1. To resolve this issue, navigate to Administration > Local settings.
2. Uncheck the box called Activate Control For Prebook Login and this issue will never appear again.
3. You can also change the timespan for when Preebooked visitors can be activated by increasing number of hours before meeting and after meeting.