jonathanm,
Can you post the following (in this order) for the "MoveVMs" instance you showed above:
1. Existing vms_session.cfg file content.
2. Output of /opt/VirtualBox/VBoxManage list runningvms from terminal.
Press STOP button in VirtualBox Plugin WEBUI
3. Syslog
4. vms_session.cfg file content (again).