I'm running FreeBSD 13.0-RELEASE in VirtualBox with virtualbox-ose-additions-6.1.18_1 and virtualbox-ose-kmod6.1.18_1 and whenever I start Plasma, ether through sddm or xinit, it stays stuck on the loading screen. When I disable vboxguest and vboxservice then Plasma is able to start.
If you have not already done so:
1. stop the guest
2. set the guest to use VBoxSVGA
See if it works with guest additions for a user who is not a member of wheel.
If so, then it's probably the same issue described here:
(In reply to Graham Perrin from comment #1)
this was the issue, switching graphics controllers worked perfectly
(In reply to kam from comment #3)
Thank you for confirming.
Related: bug 254266 comment 7 – I'm to edit the package message, maybe later this week. For fewer users of emulators/virtualbox-ose-additions to have the pitfalls of VMSVGA …