What is the output of the following command (to be executed in a terminal)?
$ sudo apt update
And this one?
$ cat /etc/apt/sources.list
Note: you can select the text in the terminal and then middle-click in the
text field of this forum (or click at the same time the left and right
buttons if your mouse does not have a middle button).
About "Backup", where do you backup to? A removable drive? If so, does the
error window pops up when you plug it? Try closing that window and opening
the "Backup" utility.
Finally, your system monitor may show, by default, only the user processes.
Either find out how to show all processes or use 'top', in a terminal.
