How to get rid of the response alert ("red rectangle")?

I now have another case where the alert is lit, and I do not know a way to switch it off.
I cannot visit the thread via the link given with the response shown under “notifications” (in yellow) because the thread obviously was deleted a few minutes after the respose for unknown reasons. At least I am informed that the content no longer is available if I try.
I also cannot delete or acknowledge the notification. Marking it and calling ‘delete’ or ‘mark seen’ subsequently has no effect at all.

There is a problem with alerts.

See Why does the Notification Javascript no longer Work?

Thanks! Do you also know something about vanishing threads? This was the second time that I experienced a thread linked in a notification but already no longer availabel. The reasons were not understandable for me in both cases. The first time at least I could clear the notification finally.