Sorry to bring up a dead thread, but I had this exact issue today (1 day after installing and multiple reboots of it working flawlessly). Screens show at login, but my second display shows no signal after logging in.
Luckily found this answer from 2015 that resolved the issue. Delete the folder ~/.local/share/kscreen/ then reboot.
I had to remove
~/.local/share/kscreen/on plasma5