Invitations that are in "open invitations" are not correctly handled in email invitations.
Declining an invitation results in "Error: Could not find incidence to delete." and accepting it results in a second event being created in the default calendar.
This is likely due to the event being filtered from the default calendar as long as it's not accepted (since it shows up in the "open invitations" folder).
We should likely use a different way to retrieve the event to update/remove.