The first step in diagnosing an app crash is always to check the exception stack trace. You can find it in the logcat. Post it here in its entirety. It includes the line number of the offending code, giving you a good start with debugging.
Oracle has endorsed the GraalVM as an alternative to Nashorn. My favorite would be the time-tested Rhino library, which is now a standalone library. I understand that Nashorn can have better performance, though, so that may not be an option.
If the code is thread-safe (which means protecting access to shared mutable data) then it doesn't matter if the code is run concurrently for multiple users. Not using instance variables (at least none that change over time) in your servlet and filter would be a good start.