As far as I checked, our favor DE MATE doesn't met the requirements of the last testing GTK3 version and it seems to be a great battle for the future of MATE.
I'm waiting for the new, corrected MATE, stable and issue-free as we knew it and love it.
Till then, Sparky support 20+ options for a stable DE, such as LXDE if youre looking for a light DE, XFCE, Cinnamon etc.
You just have to go to APTus Desktop, make your choice and install it. Then logout and login choosing the new-installed desktop. You can login back to MATE after upgrades to check it's progress.
If you prefer to stay in MATE but caja doesn't work, you can install pcmanfm, the lightweight file manager of LXDE.
Testing new software sometimes getting hard. usually with a happy end.