"VboxManage debugvm dumpvmcore" error

I'm trying to make an .elf file with VboxManage.exe using the command:

C:\Program Files\Oracle\VirtualBox>VBoxManage.exe debugvm "win8.1" dumpvmcore --filename="%userprofile%\desktop\mem.elf"

I'm getting the errors:

VBoxManage.exe: error: Code E_FAIL (0x80004005) - Unspecified error (extended info not available)   
VBoxManage.exe: error: Context: "LockMachine(pArgs->session, LockType_Shared)" at line 798 of file VBoxManageDebugVM.cpp

The machine is running.

The machine was copied from another computer but works fine with no errors when I am running it normally with VirtualBox.

Does anyone have any idea what the solution for this is?