This is very frustrating!
I upgraded to 4.1.10, booted the VM and received a message about old guest additions. For the first time ever, instead of mounting the iso from within the VM OS I used the "install guest additions" options in the mini menu bar. This failed with "Cannot attach medium 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso" so I just mounted the iso and ran the script as I normally do. Now after installing guest additions and building kernel modules I shut down normally. Today I start VBox and the machine is inaccessible!
Cannot attach medium 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso' {d6c41d7d-3727-4845-9aad-d89f671f7dc7}: medium is already associated with the current state of machine uuid {a2844f20-fc58-4d8b-8d01-d25a0358c487}!.
Result Code:
VBOX_E_OBJECT_IN_USE (0x80BB000C)
Component:
Medium
Interface:
IMedium {53f9cc0c-e0fd-40a5-a404-a7a5272082cd}