The recent VMware Workstation Pro 16.1.0 update application causes Windows 8.1 Pro to start up incorrectly via Microsoft's 'Syskey' tool. Inability to enter the password requested by Syskey and therefore to switch to Windows 8.1. No character can be entered. With VMware Workstation 16.0.0, no worries. I think the 16.1.0 update is involved in this blockage.