Default App dissapeared...needs to be reinstalled


this happened to me the very 1st time. Im using a Optimus 3D (p920) with PACMAN ROM v21.3 since many weeks now.

Its great...

But today I used my GMX mail app to send 2 photos. While I once again realized it had no network, which usually needs a restart.

After the restart, my GMX mail app was gone and checking the playstore for GMX, the app offers to be installed. My shortcut is also no longer working.

How can this happen ? Why does it happen ?