Android : Android runtime exception "Could not find class" in the same package as found classes

I get a runtime exception "Could not find class..." for a class that is in the same package as the main activity that starts running. See logcat below.

My main activity starts to run properly, then a fragment "MyFragment", which is defined in the same package as the MainActivity can not be found.

07-09 22:03:15.876: E/dalvikvm(30826): Could not find class 'com.x.MyFragment', referenced from method com.x.MainActivity.onCreate
...
07-09 22:03:16.061: E/AndroidRuntime(30826): FATAL EXCEPTION: main
07-09 22:03:16.061: E/AndroidRuntime(30826): Process: com.x, PID: 30826
07-09 22:03:16.061: E/AndroidRuntime(30826): java.lang.NoClassDefFoundError: com.x.MyFragment
07-09 22:03:16.061: E/AndroidRuntime(30826):    at com.x.MainActivity.onCreate(MainActivity.java:192)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.Activity.performCreate(Activity.java:5451)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2471)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.ActivityThread.access$900(ActivityThread.java:175)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.os.Handler.dispatchMessage(Handler.java:102)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.os.Looper.loop(Looper.java:146)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at android.app.ActivityThread.main(ActivityThread.java:5602)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at java.lang.reflect.Method.invokeNative(Native Method)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at java.lang.reflect.Method.invoke(Method.java:515)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
07-09 22:03:16.061: E/AndroidRuntime(30826):    at dalvik.system.NativeStart.main(Native Method)

FWIW, this is code that has run for a very long time, until I had to re-install Eclipse after the Google ADT 23 problems.

Please help.