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The `java.lang.NoClassDefFoundError` is a fairly generic error message and can be hard to debug correctly without further context.
It will be good to implement some form of logging for your application to help you narrow down the root cause quicker. The advantage of implementing logs with your code is that some errors might not cause a significant problem until it is too late. This way, you can monitor your running code in different situations.
How did you deploy your App? An excellent way to manage your code for App Engine is to create a repository and deploy an app to App Engine directly from the repository. This way, you gain complete control over any changes and even automate smoother deployments.