I've been struggling to use VBoxManage copyfrom on my Ubuntu VM for a couple hours with seemingly no similar cases online, so I decided to post my solution if anyone else searches for it.

The error I encountered:

VBoxManage.exe: error: The specified user was not able to logon on guest
VBoxManage.exe: error: Details: code VBOX_E_IPRT_ERROR (0x80bb0005), component GuestSessionWrap, interface IGuestSession, callee IUnknown
VBoxManage.exe: error: Context: "WaitForArray(ComSafeArrayAsInParam(aSessionWaitFlags), 30 * 1000, &enmWaitResult)" at line 879 of file VBoxManageGuestCtrl.cpp

Turns out, the credentials were off because somewhere the uppercase letters were ommited.

After changing the username/password to lowercase (and turning automatic login on for good measure) I managed to run it normally.