edit: this issue is solved by installing dconf-editor
Mate 1.28 just landed in the repo today and upgrading has resulted in broken panel applets. The only applet I use that works currently is the classic menu.
Applets I use that are broken:
clock
notification area
battery
weather
window list
If MATE 1.28 just landed in the repos, then the applets will be updated, if and when the maintainers get around to it. You’ll just have to be patient until then.