One uses Microsoft Edge, version 42.17134.1.0 with the Vector skin and reports that the problem has been appearing periodically for months.
Another one uses Google Chrome, version 63.0.3239.108 with Monobook. The problem appeared two days ago.
A third one uses Google Chrome too, version 75.0.3770.100 with Monobook. His case is a little different, since he does not receive any notifications....
I will update this message based on feedback.