[SOLVED] Cinnamon update broke desktop background

[Erathiel]

Hello

Today I updated Cinnamon on my LMDE2 from version 3.2.7 to 3.4.3, then rebooted. Everything seems to be working fine except for desktop background. I have an external display connected to my laptop via VGA. The problem is the wallpaper is only shown on the external monitor, while on my laptop screen the background is plain grey. It doesn't change when I switch wallpapers or picture aspect. When I disconnect the external screen, the background wallpaper is shown in my laptop screen but after connecting it back again, the situation reverts to the initial state, i.e. wallpaper on the external screen, plain grey on laptop screen.

Screenshot with the external monitor connected:

Please disregard the black rectangles, this is intentional

What can I do to debug this?

[Erathiel]

The funny thing is that if I lock the screen, the wallpaper is visible on both displays. Weird.
I've also checked switching to another user and it works fine, the wallpaper is visible on both displays.

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Mysteriously fixed by updating Cinnamon to 3.4.4.