Skewed Log In Screen When Screen Sharing
Reconnect the monitor.  On the Mac Pro, set the resolution to a lower setting (as a test to the lowest possible one).  Then try again with the monitor disconnected.

If it works, you can then increase the resolution.

I've had similar problems happen several times over the years.  Sometimes connecting a display adapter to the video card (even though it is not connected to anything) will fool the video hardware into working properly.