After a VirtualBox upgrade, a previously saved VM may report a "Failed to open a session for the virtual machine..." VirtualBox Error. Choosing Details shows a message similar to "Device 'serial'/0 not found (VERR_SSM_LOAD_CONFIG_MISMATCH)."
This is caused because newer VirtualBox versions handle a missing Serial port differently. Older versions would silently continue when the host machine did not have a physical serial port (like COM1). More recent versions of VirtualBox have instead notify the user with this error. The fix is to remove the Serial Port from the Virtual Machine configuration while the VM is Powered Off. The Serial Port is not required for the Virtual Machine to operate, so it can be safely removed from the configuration.
To Disable Virtual Machine's Serial Port:
1. From the Oracle VirtualBox Manager window, select the VM. If the VM is "Saved", then first choose Discard Saved State from the VM's right mouse button menu or the Machine menu.
2. Choose Machine > Settings... then choose Serial Ports
3. Disable the Enable Serial Port check box.
4. Choose OK to close the Settings window.
Figure 1: VirtualBox Error
Figure 2: Machine Settings > Serial Ports