Yes, this is it. Edit your ~/.xinitrc replacing startkde with startplasma-x11, then startx should run fine.
Everyone who has Manjaro Plasma for a long time should check their ~/.xinitrc for a correct command starting Plasma. It should be startplasma-x11, not startkde. Or better simply do cp /etc/skel/.xinitrc ~/.xinitrc.
Also my recent research on “black screen with mouse cursor” ended up with a conclusion that it may be caused by Night Color fail to detect user’s location, so it should be set to custom location to speed up loading of Plasma desktop.