Same problems as described in
CORE-7670 and in CORE-10077, but using ReactOS r69754 on VBOX 4.1.18 r78361 (and NO, the correct fix is not to upgrade to a more recent version of VBOX, but it is to really fix USB. Because Windows with its own USB drivers work fine on this version of VBOX).
A way to reproduce is to un-focus the VM window (by clicking elsewhere...) or detaching keyboard/mouse using the host key combination, while ReactOS is loading the USB drivers. It looks like a synchronization problem for me.;;