Update:
I moved this thread to the virtualbox-bin comments, https://aur.archlinux.org/packages/virtualbox-bin/#comment-702048:
Ralf_Mardorf commented on 2019-07-27 17:50 (edited on 2019-07-27 17:51 by Ralf_Mardorf)
Hi, a Windows 7 guest quite suddenly can't access a shared folder anymore. If I open the shared folder, I get a BlueScreen and Windows 7 immediately crashes and restarts, see [aur-general](https://lists.archlinux.org/pipermail/aur-general/2019-July/035220.html). In the meantime I tested 3 different kernels and restored Windows 7 from a snapshot, as well as Windows 7 and ~/.VirtualBox/ from a backup. Windows doesn't provide useful information, but the VirtualBox logs mention "Failed to open "/dev/vboxdrvu", errno=13, rc=VERR_VM_DRIVER_NOT_ACCESSIBLE", which seems to lead to a vboxdrv.rules issue. However, even after getting rid of the "Failed to open "/dev/vboxdrvu", errno=13, rc=VERR_VM_DRIVER_NOT_ACCESSIBLE" message, by using vboxdrv.rules from the community package, the Windows guest still crashes, see [aur-general](https://lists.archlinux.org/pipermail/aur-general/2019-July/035221.html).