Crash when using SystemInstaller

I've installed the app into /system/priv-app, and enabled system installer. I've android 4.4.4. Btw., even when I disabled unknown sources, tradidional installation works, when the app is in the priv-app directory.

The important stacktrace from the log seems to be:

E/JavaBinder(2855): *** Uncaught remote exception!  (Exceptions are not yet supported across processes.)
E/JavaBinder(2855): java.lang.AbstractMethodError: abstract method not implemented
E/JavaBinder(2855):  at android.content.pm.IPackageInstallObserver$Stub.packageInstalled(IPackageInstallObserver.java)
E/JavaBinder(2855):  at android.content.pm.IPackageInstallObserver$Stub.onTransact(IPackageInstallObserver.java:56)
E/JavaBinder(2855):  at android.os.Binder.execTransact(Binder.java:404)
E/JavaBinder(2855):  at dalvik.system.NativeStart.run(Native Method)
W/dalvikvm(2855): threadid=22: thread exiting with uncaught exception (group=0x41710ce0)