There seems to be an issue with Synergy and the i3 window manager, when after logout the screen just turns blank and there is no way to log on to the sytem, anymore…

This bit me several times, and all I could do is rebooting the machine – until I found out I could restart the Gnome desktop manager, which is used for showing the login screen.

Later, I found an article – Fix Can't Login After Logout With Gnome – pointing me to the actual issue, a line added by Synergy to a GDM config:

# /etc/gdm3/PostSession/Default
systemctl --system start synergy.service

I removed this line, and logout works again. I contacted Synergy about the issue, they friendly replied and want to investigate.