We are at intermittently unable to access data within spreadsheets due to an issue with Google's systems. This is preventing us from sending some notifications and causing errors to be shown in the add-on occasionally.
We have reported this issue to them and are awaiting an update. Notifications should still be sent eventually but may be delayed for an unknown amount of time. Apologies for any inconvenience this may cause.
We will post an update when the situation changes.