APP says "package file has bad manifest" when installed from market

I have an app that works fine from emulator or on device testing, but when ins开发者_JAVA百科talled from the market it can't install with the fllowing error: "package file has bad manifest"

what could be causing this? I've been checking through the manifest dozens of time without results.

[EDIT] Here is the manifest with some sensitive parts edited

<?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android" 
              package="com.xxx.xxx" 
              android:versionCode="7" 
              android:versionName="1.55">
  <uses-sdk android:minSdkVersion="7" android:targetSdkVersion="8" />

  <supports-screens android:resizeable="true" 
                      android:smallScreens="true"
                       android:normalScreens="true"
                      android:largeScreens="true"
                      android:anyDensity="true"/>

  <application android:label="My App"
               android:icon="@drawable/icon"
               >
    <activity 
          android:label="My App"        
         android:screenOrientation="portrait"
                 android:configChanges="keyboardHidden|orientation"
          >

      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>
    <activity android:name="com.google.ads.AdActivity"      android:configChanges="keyboard|keyboardHidden|orientation"/>
  </application>
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
</manifest>

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