v4.5 "Slaine" Update Manager cannot fetch packages

I just installed Trisquel v4.5 "Slaine" on my old IBM Thinkpad X23 Laptop to revive it, and have some useful joy again.

However, when updating with the Update Manager in the GUI, it failed to fetch some files from "slaine-updates main".

sysvinit-utils i386 2.87dsf-4ubuntu.19.1
sysv-rc all 2.87dsf-4ubuntu.19.1
initscripts i386 2.87dsf-4ubuntu.19.1

I worked around the problem by pressing the triskelion logo button in the lower left corner and choosing Accessories -> Terminal. In the terminal I then write

sudo apt-get update

Write my password and press Enter/Return, wait for the command to finish, and then write

sudo apt-get upgrade

And wait again. The last command should finish with some messages about processing triggers.

That's it.