Comment # 7 on bug 1176229 from Dirk S.

That's it, thanks!
Here is a diff of both kwinrc files (< without borders, > with borders)
6c6
< OpenGLIsUnsafe=true
---
> OpenGLIsUnsafe=false
9,10c9,10
< Id_1=20594073-f966-4c01-b85b-66de536db94b
< Id_2=a2e0bfbd-a225-4659-8cda-517de34047bf
---
> Id_1=fba2f652-b464-42dc-b918-0bba59e25d24
> Id_2=f3b9e745-34d8-4cad-8c12-4f83b1a023d8
11a12,24
> 
> [Effect-PresentWindows]
> BorderActivateAll=9
> 
> [ElectricBorders]
> TopLeft=None
> 
> [TabBox]
> BorderActivate=9
> 
> [Windows]
> ElectricBorderMaximize=false
> ElectricBorderTiling=false