I'm using Nova Launcher. If not the default MIUI Launcher is selected, you get an error.
After changing lockscreen wallpaper you can switch back to Nova launcher.
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As workaround use MIUI Launcher as standard launcher shortly to change the Lockscreen Wallpaper. After that you can change back to the other Launcher.
Its a MIUI bug, really getting on my nerves.
Gesendet von meinem Androiden mit Tapatalk