5.10 Emulator doesn't start after first run

First time emulator boots up ok but after shutting it down I can't start emulator anymore. Error message I get is:

Error message from VirtualBox:

Oracle VM VirtualBox Headless Interface 5.1.30_Ubuntu
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All rights reserved.

VBoxHeadless: error: The machine 'Boot2Qt-5100' is already locked for a session (or being unlocked)
VBoxHeadless: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
VBoxHeadless: error: Context: "LockMachine(session, LockType_VM)" at line 946 of file VBoxHeadless.cpp