Why to i keep getting ''Installation Blocked'' messages?

I have an experia Z3 with lollipop 5.1.1 . For the last few weeks or so, i've been getting pop-ups saying ''Installation Blocked''.
It started with apps I don't really use like ''Line'', never knew i had that.. But now it started to do the same thing over and over again. And now with trusted apps that i got from Google Playstore like Instagram and Twitter. All the apps are working fine, but I keep getting this annoying pop-up each 30 minutes or so... Why would you say installation blocked, if the app works just fine? It also say to check the settings to Authorize installation of applications from unknown sources. Which is weird coming from apps like IG and Twitter that are known and trusted apps... I just find it weird that they ask for me to change the settings and allows apps from unknown sources.. I also have AVG protection, but since I phone for a while now, I don't think this is the problems.. Would anyone have a solution or had that problem before?
P.s. Sorry if I made mistakes, i'm a French guy!

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