Scheduling Notifications when saving a meeting

Hi,

I have a customer that uses Microsoft Dynamics CRM 2016 On-premise 8.1 version.

When saving an appointment we are getting Scheduling Notification.

Translation from Hebrew- "The following possible problems were found in the schedule. You may not be able to schedule this activity until these problems are fixed"

It happens every time after saving an appointment!

Is anyone familiar with this problem?

Thanks,

Eran