Same, from time to time (like every 1 or 2 days) my 9T pro is doing the same thing. Always restarted when I tried to unlock it.
I enabled some system apps that I disabled (family guard etc) because i read on xda that it might be the cause.
Can confirm it on my 9T Pro. It just says No apps that support this format when trying to install any APK from Downloads app
Rom: latest stable MIUI 11 rom
EDIT: Whats weird, it seems that only apps that are somewhere in /data folder can't be read. Can't install from Dropbox but can from Google...