Monitor Resolution Too Low

I purchased a lifetime membership in July and carefully selected all hardware to meet the specifications required, and it has all been working perfectly until mid-today when the core told me the screen attached to my Mac Mini core had too low a resolution to run Roon. I have a ViewSonic 18.4-inch (1366 x 768) screen that has worked perfectly since July which I bought specifically for the core. Even if I run in full screen the bottom is cut off. Help please. Thank you. Ps. LOVE the software!!!