Richard,
When the host clicks the cancellation button, one of the penalties Airbnb imposes is that the days that you have cancelled are automatically blocked. Could it be that the guest requested a cancellation, and you, trying to be the helpful host, went in and made the cancellation on your guest's behalf? Airbnb still counts that as a host cancellation.
I ask this because I noted on your profile that you had an automated posting in May that the host cancelled the reservation, and you responded that the guest had.
In either case, your best bet is to contact Airbnb to correct the situation: Either the guest initiated the online cancellation process and your nights are erroneously blocked or you cancelled on behalf of your guest and Airbnb's automated systems kicked in. The conversation you've had with the guest through Airbnb should clear that all up.
Good luck!