Hi @i_rina,
Thanks so much for being the only person so far to take time to help.
I looked at the articles you mention and none of them seem to help. I would understand if the + sign to add Spaces were not visible because it was offscreen because of a resolution or overscan issue. And I manually adjusted screen resolution to ridiculously small sizes just to see if it helps. Oddly, everything works perfectly when connected to my MacBook Pro and MacPro. The problem seems to be specific to this Mini.
And on the Mini, the dock is visible on the right of the monitor as are the controls for Notification Center: so the entire screen is visible. Just no + bit for Spaces.
Since the Mac mini is less than a week old I'll schedule a visit to the Apple Store and waste a Genius' time with this. I'll post back here with an answer.
Thanks again for taking the time to offer help.