(In reply to Marek Olšák from comment #23)
> The xorg ati driver is for the radeon kernel module.
> The xorg amdgpu driver is for the amdgpu kernel module.
> 
> If the xorg driver is missing for the respective kernel module, the X server
> will use the generic xorg modesetting driver instead. The behavior might be
> the same because the modesetting driver has similar capabilities as the
> hw-specific drivers.

My mistake...I  took an other look on the system, and the KDE scale method was
set to smooth (The default is accurate). This is reproducable 
System > compositor > scale method > accurate = crash
System > compositor > scale method > smooth or crisp = Can Alt + Tab without
crash.
So you are right, the different X11 driver doesn't solve the problem.