The error is coming out of the Chaining Engine. If you have not wiped the actual error message, it looks as if the reference is to a non-existent or empty rule.
If this is happening only in Production, and every time you close any work object (is that true? Does it happen for all work objects?), then I'm guessing something is broken in the Declarative cache.
I would run the SMA tool and go to Advanced >> Declarative Rules Management, and click on the button "Clear Declarative Networks." This will toss everything that is in the cache and will cause the cache to be rebuilt. If the problem is due to a corrupt cache, in turn due to some unusual event, this should clear the error and allow you to rebuild a clean cache.
I would do this at a time when the system is dormant, as I don't know what will happen if you clear the cache while it is being very heavily used.