I have also this error on VBoxSVC.log :
00:00:04.465790 ERROR [COM]: aRC=VBOX_E_OBJECT_NOT_FOUND (0x80bb0001) aIID={edba9d10-45d8-b440-1712-46ac0c9bc4c5} aComponent={ExtPackManagerWrap} aText={No extension pack by the name 'Oracle VM VirtualBox Extension Pack' was found}, preserve=false aResultDetail=0
00:00:04.490793 ERROR [COM]: aRC=VBOX_E_IPRT_ERROR (0x80bb0005) aIID={85cd948e-a71f-4289-281e-0ca7ad48cd89} aComponent={SessionMachine} aText={Saved screenshot data is not available (VERR_NOT_SUPPORTED)}, preserve=false aResultDetail=0