I'm having the (basically) same problem with Ubuntu 16.04.
When it starts doing it (after a few days without), it won't let me even close the reminder window.
My solution is to unsubscribe to the calendar (through Office360) and resubscribe.
I'm using:
Lightning 4.7.8 ExQuilla 45.0.116 Exchange EWS Provider 3.9.0
I'm having the (basically) same problem with Ubuntu 16.04.
When it starts doing it (after a few days without), it won't let me even close the reminder window.
My solution is to unsubscribe to the calendar (through Office360) and resubscribe.
I'm using:
Lightning 4.7.8
ExQuilla 45.0.116
Exchange EWS Provider 3.9.0