All the virtualbox packages 6.1.34 have been removed from the build repo, this version is completely broken. It is effecting all distributions and many bug reports are open. No real fix is up, though it looks like they will come with a new release soon, see:
https://www.virtualbox.org/ticket/20904
As always when on build, and packages are removed, run pacman -Suu after pacman -Syu.