Plates are showing, but when I click the target they disappear.

I do. I will try disabling it now :D

Do you have dynamiccam by any chance?

I believe this is caused by a setting under one of the situations - probably npc interaction.

It seems to work a bit, but they still disappear.

Well, i need more information then. What do you mean "disappear"? Do the default blizzard floating names appear? When you say click the target, does it also happen if you use tab targeting (not suggesting that as an alternative, just need to know more to narrow it down). Etc.

Blizzard floating names is not appearing.
I have three targets in front of me with plates showing, I target one and use a spell on him, the plates disappear. The same is with a killing blow.
Sometimes if I use Blink when plates are showing, they disappear.
The same effect happens with tab targeting.

OK, could you enable scripterrors (/console scriptErrors 1) for me and let me know if any pop up

/console scriptErrors 1 Does not seem to be doing anything.
If I engage a target without the plates showing, they appear again.

Okaaay, do you perchance have "combat action: hostile" set to "hide" in /knp > general?

Actually that's probably not it if they're showing when you engage them. Final question for now then: which version are you on? (Its at the top right of /knp)

Hostile is set to Show. I will try to make a video of the problem.

Hmmm, I put it to Do nothing and it's working fine now. :D