Looking at the stack trace, it looks to me like you have modified the process definition, and done something "naughty" like removing one of the steps. When your existing process tries to continue, it's trying to perform a particular step, and that step is no longer there, so it throws an exception. If this guess is correct, then there is probably no way to fix it - just terminate the process, and start again.
Process modification "rules" for processes that already have active running instances are:
- Never remove any nodes or routes
- Changing properties or route-expressions are generally ok.