I've installed the latest Manjaro KDE edition with the proprietary nvidia driver(384). Kwin's compositor often just crashes while locking the screen after the timeout and leaves me with a black screen and a mouse cursor and I can't do anything. I need to kill kwin_x11 in another tty and restart it to make the desktop working again.
Please provide a backtrace of the crashing KWin. Though this sounds a lot like an issue in the proprietary driver. If that is the case I fear there is nothing we can do. But we can only investigate with a backtrace. Please note that you might have to compile KWin yourself. Arch does not provide useful backtraces out of the box, so any backtrace by Arch/Manjaro would be useless if it is not self compiled.
*** Bug 387220 has been marked as a duplicate of this bug. ***
I'm using this driver for a few months(with arch, ubuntu + various DEs) and I never had issues with compositors. Also, I've experienced kwin crashes across multiple versions of the nvidia driver and across multiple distributions. I highly doubt it's an issue with the blob.
This is probably a duplicate too - the bug with Qt and nvidia.