Plasma 5.8 from backports: can't type into authentication dialogues!

Workaround 1:

1. Go to Application Launcher (Start Menu) -> Applications -> Settings -> Set Keyboard Input Method. Then, a message will come up: 'Current configuration for the input method: ...'.
2. Select OK. Then, a new message will come up: 'Do you explicitly select the user configuration? ...'.
3. Select Yes. Then, a list will come up.
4. Select 'default' - 'use cjkv mode set by /etc/default/im-config'. Then, a new message will come up: 'Setting the user configuration /home/user/.xinputrc to xim. ...'.
5. Select OK.
6. Restart KDE session (log-out and then log-in to KDE).

Workaround 2:

1. Go to Application Launcher (Start Menu) -> Applications -> Settings -> Set IBus Preferences. Then, a message will come up: 'Keyboard Input Methods (IBus Daemon) is not running. Do you wish to start it?'
2. Select Yes. Then, a new message will come up: 'IBus has been started! ...'.
3. Select OK.

If workaround-2 works for you, then, you will have to repeat it, in every KDE session (after every log-in to KDE). So, workaround-2 is not permanent.