Xamarin Forms app crashes in release mode

My Xamarin Forms app crashes in release mode. The Android log says:

"Info(1209)/ActivityManager: Start proc com.companyname.testapp for activity com.companyname.testapp/md52b7e1ecdf8804bc8b10ae2984b6a4185.MainActivity: pid=32642 uid=10253 gids={50253, 9997} abi=armeabi-v7a
Warning(32642)/monodroid: Trying to load sgen from: /data/app/com.companyname.testapp-1/lib/arm/libmonosgen-2.0.so
Error(32642)/mono: The assembly mscorlib.dll was not found or could not be loaded.
Error(32642)/mono: It should have been installed in the `/Users/builder/data/lanes/1978/f98871a9/source/monodroid/builds/install/mono-armv7/lib/mono/2.1/mscorlib.dll' directory.
"

This happens on a Sony Xperia Z3+ with Android 5.0.2. However, it works fine on a Samsung Galaxy S3 with Android 4.3.

This even happens on an empty project created with the Xamarin Studio wizard.

Any ideas...?