Today I have noticed that I can't get rid of the notifications. The "Close" button doesn't seem to work no matter how many times I click on it.
At first I saw this manifesting while I was browsing using Chromium.
Then I tried Firefox - the notification was still there.
The same thing happens with other browsers I got: Opera, Midori, Vivaldi. And even with other apps.
I saw that there is an apparent duplicate of this question on AskUbuntu, but the situation is obviously different.
Also, on a Google search I couldn't find anything similar except for this question on the Windows website. Apparently, in that case, the cause of all this was a malware. But I am using Ubuntu 16.04, which, as far as I know, is not (that) affected by malware.
So what could be the cause? Can you, please, tell me how to fix (and prevent) it?