Initially it was having problems downloading or updating apps. Showed as if it were downloading but it actually wasn't it was doing anything, it was just stuck on the animation. Even after a reboot it was still present, but I think it's fixed as of now.
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That happens to me sometimes, what I've sort of pinned it to, is miui being so intent on you using wifi instead of data for absolutely everything.
Disabling the download size limit in the download app doesn't really fix this either..
Because every now and again when you download stuff, a dialog box will pop up asking to queue it or cancel it, (this is different from the 'set up wifi' box that appears) and I find that sometimes it just disappears and will leave your download, and all subsequent downloads, in limbo.
What sort of helps is raising the limit miui thinks your data allotment is per month from their data monitor app (called 'network assistant') .
Also, in the settings in that app, there an option three down from the top, as seen in the screen shot, (the "disconnect from the Internet...'" option) that helped out when I disabled it..
I was having download problems and after Disabling it, the problem seemed to be fixed...
This might be unrelated to your problem, But I figured it might help..
Sent from my SGH-T999 using xda app-developers app