I will test the tainlog CVar in my next play session.
What I found out during my last session was that this problem only occures in a PvE environment. It never happened during PvP. It got me thinking that it only happens when I fight NPCs.
I never spotted the failure message though. It always happens mid-fight so my attention is elsewhere. I will just brute force a enable/disable addon method and try to catch what addon is causing it as there is no typical Lua error message appearing.
Yes in my posted screenshot, the unit is indeed my current target. But this effect also applies to other nameplates that are around. It usually starts with the target getting changed, nameplates that spawn afterwards then show the same behaviour. You can see this in the screenshot aswell, "Tugar Bloodtotem" and the one "Bile Spitter Egg" spawned before hand and are normal while the other egg changed it's alpha. From now on, every newly created nameplate will have the changed alpha.
For the dark UI i used
Santa UI Textures. But at the moment I have it disabled because I suspected it to be responsible for the nameplate issue, but nothing changed after I stopped using it.
The other way you can do it is by changing the texture color of the Blizzard UI. This way is used in
Lorti UI and I also use this method in my "Scripts" addon.
The actual addon "Scripts" does a bit more then the nameplate stuff. It sets up my whole UI. This is how
Scripts looks like. It's kinda big but I think it's easy to find your way around as I tried my best to structure and comment it in a good way. You can find the skinning of the UI in there aswell as all the other stuff i personally changed so maybe the reason lies in here somewhere.