poppler-qt5 is being replaced by poppler-qt, not clear why Octopi fails to automate the replacement this time, I will see if the Octopi dev can help out fixing this.
In the meantime, Octopi is just a GUI for Pacman, use Pacman directly:
sudo pacman -Syu
and it will ask you to replace poppler-qt5. (with the default being 'Yes')
https://kaosx.us/docs/pacman/