I’ve seen same effect on two different headless servers recently upgraded to 18.04, and using the Mate Desktop.
Even created a brand new account to see if that was issue – no.
However, after “fiddling around” with resolutions, backgrounds, and reboots, it has gone away on both.
I think it’s because the NoMachine client can’t find the right resolution…
Try CntrlAlt0 and force other resolutions, use Displays to set other resulting, and keep trying!
By the way, I’m using NoMachine to create a virtual desktop, as there is no monitor, and Lighten is set to seat default desktop false
Hope that helps