I use KDE on Debian. I have not encountered this, nor can I think of a reason why showkey would break a user's desktop session.
If the GUI login screen is still visible when it hangs, I suppose sddm might be having trouble. To investigate, I would run journalctl -f in a text console, and maybe tail -F /var/log/Xorg.0.log* in another, while attempting a GUI login. When it hangs, I would switch back to the text consoles and see if the most recent log messages hint at what's hanging.
*(Or whichever log file corresponds to the new X session, assuming you're using Xorg instead of Wayland.)
Could the fingerprint reader be causing the problem on the main account?