I encountered this exact problem after a recent update of the ADT component of Android. It looks like Android is ignoring the build path and instead using the ANT defaults.
I solved this by removing my jar file from the build path, creating a folder in the “root” of the app heirarchy (alongside src, gen, etc) called “libs”, and putting my .jar there. When you clean / build and re-launch the app, the error should go away.
FYI – this is occurring because the JAR file is not getting packaged into the .apk files – this is why it builds correctly, but isn’t available during run-time on your Android device.