KWin crashes

Good evening,

Just did a fresh install, set up xorg, nvidia drivers and Plasma 5 and everything worked fine...
...until that moment I created a new user and deleted the one I managed to get Plasma to work on.
If I try to start Plasma with this new user, the KDE splash screen does appear but the screen becomes blank right after with only a mouse cursor, and I eventually have some flashes of the actual plasma desktop.

Yet I can start twm without problems, so neither xorg nor my nvidia drivers are to blame. I can even do a 'startkde' from twm, which launches plasma but with twm instead of kwin as the window manager.
But in that case, as soon as I enter 'kwin_x11 --replace', I fall back into the same problem.

So that lead me into thinking that kwin was the problem, but I have no idea on how to fix it or what to look at to get some interesting verbose...

Thanks in advance for your welcome help