Then the possible, theoretical issue does not exist.
With most distributions it is recommended to use the guest iso to install certain support files.
This is not necessary with Manjaro Linux. The required package will be provided during installation (by mhwd).
The forum has an extensive troubleshooting and configuration guide in the Contributions > Tutorials section
[root tip] [How To] VirtualBox - Installation - USB - Shared Folder
What comes next is speculation…
Manjaro Plasma default to use Wayland - I don’t know if this is an issue, because I have never purposely tested for that scenario.
You can however test it - instead of signing in - you can use the Host-key+Fkey (if I recall correct) to switch to a TTY in the virtual machine.
Sign in and sync the system while adding the plasma-x11-session.
sudo pacman -Syu plasma-x11-session
Then restart the system - and in the lower left corner change session type to Plasma X11 - and sign in.