I asked this question 6 years ago. In the meantime Android development best practices have changed, and I have become a better developer.
Since then, I have realized
In my case, the AndroidManifest.xml of my application (kick-started by the default Eclipse assistant) contained android:theme="@style/AppTheme" in the block.
When debugging the cause of the problem, it turned out that the line
mMethod = c.getMethod(methodName, PARAM_TYPES);
in android.view.MenuInflater/InflatedOnMenuItemClickListener was called with c not being my Activity class but a dubious android.view.ContextThemeWrapper (which of course doesn't contain the onClick handler).
So, I removed the android:theme and everything worked.