I'm using plasma 5.8.5 on Gentoo (w/systemd). Often, the shutdown button on the panel (from the loc.logout widget) doesn't work, i.e., the screen overlay giving the different shutdown options (restart, logout,…) doesn't appear. How can I find out what is the root cause?

Could it be you have an Intel graphic card and a not so new computer? I'm having the same issue here. And I am under the impression that it has to do with the switch from the xserver-xorg-video-intel video driver to the new xserver-xorg-video-intel-native-modesetting driver.
Does it help when you re-install xserver-xorg-video-intel driver?

(sudo apt install xserver-xorg-video-intel)
I keep on looking too.
Pieter.

No, I don't think the video driver is causing it. After switching a few times.
I can use the kickoff menu to shut down and restart again today but it doesn't show me the shutdown options before it shuts down/restarts. In the system preferences I told kde to do so but it doesn't.
Perhaps it is a separate widget, the shutdown/restart thing. Perhaps that widget is not good. But I don't know how it is called.

Further I noticed that yesterday the programm "lp-solve" was removed. I reïnstalled it and since then I can shutdown/restart from the menu again (but it doesn't offer the options). Still I very much doubt if lp-solve has anything to do with this issue. You could try...

Now, as I look again, the shutdown and reboot buttons in the menu don't work at all anymore. I have no clue.

I solved it!!
I happen to "dual boot" my Neon session with Ubuntu on this machine. What I did was the following:

from ubuntu I deleted much of the hidden files of my Neon home-folder. Carefully leaving the .thunderbird and .mozilla folders in tact. It would take some time to restore them and I didn't think these folders would be relevant to the problem.
After that I restarted Neon and the problem is gone!
I restored my desktop preferences "et voilá".

I imagine you can also install the Midnight Commander (sudo apt install mc), start it (from the menu or from a terminal), and then shut down plasma: (from a terminal "killall plasmashell"). Then you can delete (hidden) files and folders from you ~/home folder. The more the better.
When you think you deleted enough (.config, .local, other hidden files etc) restart the system and the shutdown/restart buttons will work again.
Somewhere in the hidden files something was wrong. But I don't know where.
I hope it will work for you too!

begonia wrote:Then you can delete (hidden) files and folders from you ~/home folder. The more the better.

This is a really dangerous approach. These directories contain useful configuration and cache data.

Anyway, for now the problem hasn't reared its head in a while.