An Exchange FreeBusy request sent from Notes fails and the following error is logged:
"Exception"System.NullReferenceException: Object reference not set to an instance of an object".
This error can appear for several reasons such as unlicensed O365 users or when an O365 user has not logged in for the first time yet.
Unfortunately this issue also affects other users when running an Exchange FreeBusy request from Notes. For instance, if you run an Exchange FreeBusy request from Notes for two O365 users and one of the them is unlicensed, the FB lookup request will fail for both them.