Detachable run won't let its window close correctly

[Daniel U. Thibault]

VirtualBox 5.0.18 r106667, Ubuntu 14.04.4 (64-bit) guest running on CentOS 7 (64-bit) host. I run the guest in detachable mode, but when I close the window and select "Continue to run in background", I get this message:

Failed to set the extra data for key GUI/Fullscreen of machine to value {}.

The object is not ready.

Result Code: E_ACCESSDENIED (0x80070005) Component: SessionMachine Interface: IMachine {f30138d4-e5ea-4b3a-8858-a059de4c93fd}

...and the guest aborts.

[Frank Mehnert]

Do you experience the same problem with VBox 5.0.30?

[Daniel U. Thibault]

I'd rather stick with 5.0.18 for administrative reasons right now. I've tried closing that same detachable VM window (on the same host) a minute ago and this time the error was:

Failed to open a session for the virtual machine Ubuntu-14.04.4-vmnet.

The machine 'Ubuntu-14.04.4-vmnet' was unlocked unexpectedly while attempting to share its session.

Result Code: VBOX_E_INVALID_SESSION_STATE (0x80BB000B) Component: MachineWrap Interface: IMachine {f30138d4-e5ea-4b3a-8858-a059de4c93fd}