EmptyStackException in JSON plugin due to concurrency
Note: The issue was found in an app that was upgraded from version 2.3.34
I created a sample application that should help reproducing the error.
The error happened opening the app (localhost/struts-json-example) in 4 different browsers / sessions.
As a workaround I have overridden the JSONInterceptor so a new instance of JSONUtil is used with every request and the issue is not happening anymore. I suppose there is a performance cost with that change but for now it works.