Problems increasing CPU on Windows guest

RHEV Version 4.3.3.7-0.1.el7
Guest is Windows Server 16 standard.
Guest agent is installed

When I try to edit the guest to increase the CPU count from 4 to 8 on the "System" tab and click OK, the menu indexes to the next tab down, which is "Initial Run", and that text is red. Pressing "Ok" depresses the virtual button, but the task never completed.
There are very few options in this tab, and none seem to be relevant nor do any of the changes I've made help.

I've tried it with the guest powered on and off with no luck.

What can be the cause?
Is there a workaround?

Any help is appreciated.
Thanks, R