Sometimes Office365 expires the authorization to your FreeBusy. This can caused my numerous reasons. Here is how email looks like if such event occurs.
It says "Someone tried to check your availability using FreeBusy but the authorization to your Office365 calendar is expired so FreeBusy was unable to check your calendar. Occasionally Office365 will expire FreeBusy's authorization to you account as a precaution. Reauthorize FreeBusy to keep your availability up-to date."
Why this happens?
Office 365 gives its customers the ability to authorize 3rd party services like FreeBusy to access certain data, in our case calendar data. This authorization is typically valid for 3 months and unless the person uses multi-factor authentication in which case it's valid for 1 month. FreeBusy renews the authorization whenever it interacts with a customer's calendar so as long as you use the service somewhat regularly you shouldn't need to reauthorize.
However the authorization can also expire if certain Office 365 account changes are done. For example if a person enables or disables multi-factor authentication or it IT resets the account password. Office 365 will revoke all authorizations as a security precaution.
Can FreeBusy do anything?
Because the revocation of the authorization is outside of FreeBusy's control there's isn't anything FreeBusy (or any 3rd party service really) can do or predict when it might be revoked. We are working on a system that periodically checks the authorization and lets you know when it happens.
What can I do?
You can reauthorize simply by signing into FreeBusy or clicking the button that says "Reauthorize".
Can my guest book my appointment while authorization is expired?
Your guests can still book your appointment but it won't reflect your correct availability, hence you can end up getting double booked. Hence it is advisable to reauthorize as soon as you receive this email.
Kindly drop a message to us, if you have more questions about this.