Maybe you’re on to something, tried rebooting, it couldn’t boot again. I’m building on a virtual machine, because i don’t currently have a machine running Manjaro.
Would it be possible to install manjaro-tools-iso on arch linux?
I am not sure about building within a VBox install but, you can build on bare metal installs that are MBR (in my experience, on my system at least)
I also use the *-git versions as well.
Are you hitting this error consistently? I have found that sometimes it just takes a couple tries (often times a reboot helps). I haven’t built KDE, so this general advice may not apply.