Right half of desktop screen inaccesible

Help! After a recent update, my Ubuntu Mate has 'gone' half screen: icons etc. will only show on the left half of the desktop screen; they disappear into thin air when dragged toward the right half.
The Panel, background, and running apps, all use the whole screen as normal; but the desktop won't allow any desktop items, ie. icons, to appear on the right half of the screen.
I've been using Ubuntu Mate for years without a problem. I'm presently running 20.04.4 LTS (with Mate 1.20.0?), on a MinisForum UM270 with AMD Ryzen 7 27000U processor and AMD Radeon Vega 3 graphics, on a 1920 x 1080 Asus VS247H 24" monitor.
Any help would be much appreciated.