Android :: Upgrading Application From 1.6 To 2.2 Emulator - Not Working

I currently found out, that my Application developed for Android 1.6 does not work on Android 2.2. I'm quite puzzled about this, because its a very simple Application which almost does nothing. The Expeption I get is like that:
java.lang.RuntimeException: Unable to instantiate application [packagename].AndApp:
java.lang.ClassNotFoundException: [packagename].AndApp in loader
dalvik.system.PathClassLoader[/data/app-private/[packagename]-1.apk]

The weirdest part is, that the Class it is searching for here does not exist and never has. The app itself does nothing than show some text in a textview. In Android 1.6 I dont get this problem and I cant see a reason, it event works in 2.1. The Manifest looks like that:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="[packagename]" android:versionName="1.02" android:versionCode="3">
<application android:label="@string/app_name" android:icon="@drawable/stop_48"
android:description="@string/Description" android:name="@string/app_name">
<activity android:name=".DefaultActivity" android:label="@string/app_name" >
<intent-filter> <action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter> </activity>
<uses-sdk android:minSdkVersion="4" android:targetSdkVersion="4"</uses-sdk>
</manifest>