I have had Duo setup for a long time successfully on Gnome 3 and GDM. The GDM login works properly and all the polkit prompts work properly as well.
I recently setup a KDE Plasma machine. I configured my /etc/pam.d/common-auth the same way that I have my Gnome machine setup. Everything in the terminal works as expected. When starting from a fresh reboot though, the SDDM login screen never prompts me for Duo after I enter my password, login then fails. I setup fail safe so I disconnect my network and login because it can’t hit the duo server. After that, whenever something needs sudo access from the GUI, the polkit prompt fails. I enter my password, and I see the message about entering a Duo code, but there is never any text field for me to enter anything in to.
I’ve found very little about this online. Does no one here use KDE Plasma?