desktop background settings not set

In XFCE 4.20 Slackware GNU/Linux 15.1-current, FreeBSD UNIX 14.2 (both up-to-date 64-bit) I can't set desktop settings such as background/gradient. You can see below I selected a different image, but set image to 'none' and replaced it with gradient, none which was applied. This seems to happen on desktops with Nvidia graphics, ThinkPads with Intel graphics (don't know about AMD graphics but maybe can test with other GNU/Linux later). They dual-boot (shared /home) Slackware, FreeBSD. In FreeBSD opening desktop settings it says 'unable to load images from folder "null": operation was cancelled'.