You see a red banner error in the incidents pane which indicates you have exceeded the 1000 incidents quota.
Each customer has a quota of 1000 incidents per day, which means that after you exceed the quota, no additional incidents will be saved but alerts will keep on working as you defined in the incident triggers.
Out of the box, each customer receives 11 predefined triggers, they are marked in Yellow as in the following screenshot:
Most predefined triggers do not have a filter, thus in large environments you might exceed the limit.
Please make sure to go over all triggers and define them according to your needs. You can disable the ones you don't need and edit the ones you do by narrow them down a bit by using the filter editor. For more information about how to create and edit a trigger alert, please refer to our article - How to add an Incident Trigger
After verifying the triggers settings, if the issue persists, please contact ControlUp support and open a support ticket.