Suddenly I was getting posts from forums to which I was not subscribed to, eg. Installing and upgrading, General problems forums. Checked and found out that I am subscribed to them. Unsubscribed and assume that the problem is solved.
I have a vague feeling that this happened yesterday late a'noon (28 April 2021, after 16:00 GMT) after I got continuous "Something is wrong with the Internet" notices.